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Difference between revisions of "1996 AHSME Problems/Problem 25"

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==Problem==
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Given that <math>x^2 + y^2 = 14x + 6y + 6</math>, what is the largest possible value that <math>3x + 4y</math> can have?
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<math> \text{(A)}\ 72\qquad\text{(B)}\ 73\qquad\text{(C)}\ 74\qquad\text{(D)}\ 75\qquad\text{(E)}\ 76 </math>
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==Solution 1==
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Complete the square to get
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<cmath>(x-7)^2 + (y-3)^2 = 64.</cmath>
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Applying Cauchy-Schwarz directly,
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<cmath>64\cdot25=(3^2+4^2)((x-7)^2 + (y-3)^2) \ge (3(x-7)+4(y-3))^2.</cmath>
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<cmath> 40 \ge 3x+4y-33 </cmath>
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<cmath>3x+4y \le 73.</cmath>
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Thus our answer is <math>\boxed{(B)}</math>.
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==Solution 2 (Geometric)==
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The first equation is a [[circle]], so we find its center and [[radius]] by [[completing the square]]:
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<math>x^2 - 14x + y^2 - 6y = 6</math>, so <cmath>(x-7)^2 + (y-3)^2 = (x^2- 14x + 49) + (y^2 - 6y + 9) = 6 + 49 + 9 = 64.</cmath>
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 +
So we have a circle centered at <math>(7,3)</math> with radius <math>8</math>, and we want to find the max of <math>3x + 4y</math>.
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The set of lines <math>3x + 4y = A</math> are all [[parallel]], with slope <math>-\frac{3}{4}</math>.  Increasing <math>A</math> shifts the lines up and/or to the right.
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 +
We want to shift this line up high enough that it's [[tangent (geometry)|tangent]] to the circle, but not so high that it misses the circle altogether.  This means <math>3x + 4y = A</math> will be tangent to the circle.
 +
 +
Imagine that this line hits the circle at point <math>(a,b)</math>.  The [[slope]] of the radius connecting the center of the circle, <math>(7,3)</math>, to tangent point <math>(a,b)</math> will be <math>\frac{4}{3}</math>, since the radius is perpendicular to the tangent line.
 +
 +
So we have a point, <math>(7,3)</math>, and a slope of <math>\frac{4}{3}</math> that represents the slope of the radius to the tangent point.  Let's start at the point <math>(7,3)</math>.  If we go <math>4k</math> units up and <math>3k</math> units right from <math>(7,3)</math>, we would arrive at a point that's <math>5k</math> units away.  But in reality we want <math>5k = 8</math> to reach the tangent point, since the radius of the circle is <math>8</math>.
 +
 +
Thus, <math>k = \frac{8}{5}</math>, and we want to travel <math>4\cdot \frac{8}{5}</math> up and <math>3\cdot \frac{8}{5}</math> over from the point <math>(7,3)</math> to reach our maximum.  This means the maximum value of <math>3x + 4y</math> occurs at <math>\left(7 +3\cdot \frac{8}{5}, 3 + 4\cdot \frac{8}{5}\right)</math>, which is <math>\left(\frac{59}{5}, \frac{47}{5}\right).</math>
 +
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Plug in those values for <math>x</math> and <math>y</math>, and you get the maximum value of <math>3x + 4y = 3\cdot\frac{59}{5} + 4\cdot\frac{47}{5} = \boxed{73}</math>, which is option <math>\boxed{(B)}</math>.
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==Solution 2B==
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Let the tangent point be <math>P</math>, and the tangent line's x-intercept be <math>Q</math>. Consider the horizontal line starting from center of circle (O) meeting the tangent line at K. Now triangle <math>OPK</math> is 3-4-5, <math>OP=8</math>, so <math>OK = \frac{5}{3}*8 = \frac{40}{3}</math>. Note that the horizontal distance from <math>O</math> to the origin is <math>7</math>, and the horizontal distance from K to Q is 4, (<math>\frac{4}{3}</math> of its y coordinate), so the x-intercept is <math>7+4+OK = 73/3</math>. The value of <math>3x+4y</math> is 73 at point <math>Q</math>. Note that this value is constant on the tangent line, so there is no need to calculate the coordinate of <math>P</math>. <math>\boxed{(B)}</math>.
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 +
==Solution 3==
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Let <math>z = 3x + 4y</math>. Solving for <math>y</math>, we get <math>y = (z - 3x)/4</math>. Substituting into the given equation, we get
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<cmath>x^2 + \left( \frac{z - 3x}{4} \right)^2 = 14x + 6 \cdot \frac{z - 3x}{4} + 6,</cmath>
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which simplifies to
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<cmath>25x^2 - (6z + 152)x + (z^2 - 24z - 96) = 0.</cmath>
 +
 +
This quadratic equation has real roots in <math>x</math> if and only if its discriminant is nonnegative, so
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<cmath>(6z + 152)^2 - 4 \cdot 25 \cdot (z^2 - 24z - 96) \ge 0,</cmath>
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which simplifies to
 +
<cmath>-64z^2 + 4224z + 32704 \ge 0,</cmath>
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which can be factored as
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<cmath>-64(z + 7)(z - 73) \ge 0.</cmath>
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The largest value of <math>z</math> that satisfies this inequality is <math>\boxed{73}</math>, which is <math>\boxed{(B)}</math>.
 +
 +
 +
==Solution 4 (Using Answer Choice + Calculus)==
 +
Implicitly differentiating the given equation with respect to <math>x</math> yields:
 +
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<math>2x + 2y\frac{dy}{dx} = 14 + 6\frac{dy}{dx}</math>
 +
 +
Now solve for <math>\frac{dy}{dx}</math> to obtain:
 +
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<math>\frac{dy}{dx} = -\frac{x - 7}{y - 3}</math>
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Set the equation equal to zero to find the maximum occurs at <math>x = 7</math>
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Plug this back into the equation that we are trying to maximize and see that we are left with:
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<math>21 + 4y</math>.
 +
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The only answer choice that can be obtained from this equation is <math>\bf{73}</math>
 +
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==Solution 5 (Lagrange Multipliers)==
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First, we move all the non-constant terms of the constraint to one side and assign it to the function <math>g(x,y)</math>:
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<cmath>g(x,y)=x^2+y^2-14x-6y.</cmath>
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Since we are trying to maximize <math>f(x,y)=3x+4y</math>, we need to solve for <math>x</math> and <math>y</math> in the system
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<cmath>\begin{cases}x^2+y^2-14x-6y=6,\\\nabla g(x,y)=\lambda\nabla f(x,y).\end{cases}</cmath>
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We have that
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<cmath>\begin{align*}\nabla f(x,y)&=\begin{pmatrix}\dfrac{\partial}{\partial x}3x+4y\\\dfrac{\partial}{\partial y}3x+4y\end{pmatrix}\\&=\begin{pmatrix}3\\4\end{pmatrix},\end{align*}</cmath>
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and
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<cmath>\begin{align*}\nabla g(x,y)&=\begin{pmatrix}\dfrac{\partial}{\partial x}x^2+y^2-14x-6y\\\dfrac{\partial}{\partial y}x^2+y^2-14x-6y\end{pmatrix}\\&=\begin{pmatrix}2x-14\\2y-6\end{pmatrix}\end{align*}.</cmath>
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To solve the original system, we can solve for <math>x</math> and <math>y</math> in terms of <math>\lambda</math> using our equations from the gradients, then substitute them into the first equation. We have that <math>x=\frac{3}{2}\lambda+7</math> and <math>y=2\lambda+3</math>. Substituting into the first equation, we have that
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<cmath>\begin{align*}\left(\frac{3}{2}\lambda+7\right)^2+(2\lambda+3)^2-14\left(\frac{3}{2}\lambda+7\right)-6(2\lambda+3)&=6\\\frac{25}{4}\lambda^2&=64\\\lambda&=\pm\frac{16}{5}\end{align*}</cmath>
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Using the solutions of <math>x</math> and <math>y</math> in terms of <math>\lambda</math> that we found earlier, we have that
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<cmath>3x+4y=\frac{25}{2}\lambda+33.</cmath>
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Because we are trying to maximize this function, we will use the positive solution for <math>\lambda</math>. Therefore, after substituting, we have that the largest value of <math>g(x,y)</math> that satisfies <math>f(x,y)=6</math> is <math>\boxed{\textbf{(B) }73}</math>.
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~qianqian07
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==Solution 6 (polar coordinates)==
 +
Completing the square gives the equation of a circle, <math>(x-7)^2+(y-3)^2=64.</math> Seeing that we would like to maximize <math>3x+4y,</math> we parameterize the circle using polar coordinates:
 +
<cmath>\begin{align*}
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x&=7+8\cos\theta&y&=3+8\sin\theta.
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\end{align*}</cmath>
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Then, we have <math>3x+4y=3(7+8\cos\theta)+4(3+8\sin\theta)=33+24\cos\theta+32\sin\theta.</math> Since <math>a\cos\theta+b\sin\theta\le\sqrt{a^2+b^2},</math> the desired answer is <math>33+\sqrt{24^2+32^2}=33+40=\boxed{73}.</math>
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- Ultroid999OCPN
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==Solution 6 (Alcumus)==
 +
The equation <math> x^2 + y^2 = 14x + 6y + 6</math> can be written\[
 +
(x-7)^2 + (y-3)^2 = 8^2,
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\]which defines a circle of radius 8 centered at <math>(7,3)</math>. If <math>
 +
k</math> is a possible value of <math>3x + 4y</math> for <math>(x,y)</math> on the circle, then the line <math>3x + 4y = k</math> must intersect the circle in at least one point. The largest value of <math>k</math> occurs when the line is tangent to the circle, and is therefore perpendicular to the radius at the point of tangency. Because the slope of the tangent line is <math>-3/4</math> the slope of the radius is <math>\ 4/3</math>. It follows that the point on the circle that yields the maximum value of <math>3x + 4y</math> is one of the two points of tangency,\[
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x = 7 + \frac{3 \cdot 8}{5} = \frac{59}{5}, \hspace{.3in} y = 3 +
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\frac{4 \cdot 8}{5} = \frac{47}{5},
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\]or\[
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x = 7 - \frac{3 \cdot 8}{5} = \frac{11}{5}, \hspace{.3in} y = 3 -
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\frac{4 \cdot 8}{5} = - \frac{17}{5}.
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\][asy]
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import olympiad; import geometry; size(150); defaultpen(linewidth(0.8));
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draw((0,15)--origin--(15,0));
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dot("<math>(7,3)</math>",(7,3),S);
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draw(Circle((7,3),8));
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line a = line((0,9),(12,0));
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line b = line((0,12),(16,0));
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line c = line((0,18.3),(18.3*4/3,0));
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draw(a^^b^^c);
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[/asy]
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The first point of tangency gives\[
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3x + 4y = 3 \cdot \frac{59}{5} + 4 \cdot \frac{47}{5} =
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\frac{177}{5}+\frac{188}{5}= 73,
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\]and the second one gives<cmath>\, 3x + 4y = \frac{33}{5}-\frac{68}{5}
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= -7.</cmath>Thus, <math>\boxed{73}</math> is the desired maximum, while <math>-7</math> is the minimum.
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==See also==
 
==See also==
 
{{AHSME box|year=1996|num-b=24|num-a=26}}
 
{{AHSME box|year=1996|num-b=24|num-a=26}}
 +
 +
[[Category:Introductory Geometry Problems]]
 +
[[Category:Introductory Algebra Problems]]
 +
 +
[[Category:Circle Problems]]
 +
{{MAA Notice}}

Latest revision as of 16:51, 22 October 2023

Problem

Given that $x^2 + y^2 = 14x + 6y + 6$, what is the largest possible value that $3x + 4y$ can have?

$\text{(A)}\ 72\qquad\text{(B)}\ 73\qquad\text{(C)}\ 74\qquad\text{(D)}\ 75\qquad\text{(E)}\ 76$

Solution 1

Complete the square to get \[(x-7)^2 + (y-3)^2 = 64.\] Applying Cauchy-Schwarz directly, \[64\cdot25=(3^2+4^2)((x-7)^2 + (y-3)^2) \ge (3(x-7)+4(y-3))^2.\] \[40 \ge 3x+4y-33\] \[3x+4y \le 73.\] Thus our answer is $\boxed{(B)}$.

Solution 2 (Geometric)

The first equation is a circle, so we find its center and radius by completing the square: $x^2 - 14x + y^2 - 6y = 6$, so \[(x-7)^2 + (y-3)^2 = (x^2- 14x + 49) + (y^2 - 6y + 9) = 6 + 49 + 9 = 64.\]

So we have a circle centered at $(7,3)$ with radius $8$, and we want to find the max of $3x + 4y$.

The set of lines $3x + 4y = A$ are all parallel, with slope $-\frac{3}{4}$. Increasing $A$ shifts the lines up and/or to the right.

We want to shift this line up high enough that it's tangent to the circle, but not so high that it misses the circle altogether. This means $3x + 4y = A$ will be tangent to the circle.

Imagine that this line hits the circle at point $(a,b)$. The slope of the radius connecting the center of the circle, $(7,3)$, to tangent point $(a,b)$ will be $\frac{4}{3}$, since the radius is perpendicular to the tangent line.

So we have a point, $(7,3)$, and a slope of $\frac{4}{3}$ that represents the slope of the radius to the tangent point. Let's start at the point $(7,3)$. If we go $4k$ units up and $3k$ units right from $(7,3)$, we would arrive at a point that's $5k$ units away. But in reality we want $5k = 8$ to reach the tangent point, since the radius of the circle is $8$.

Thus, $k = \frac{8}{5}$, and we want to travel $4\cdot \frac{8}{5}$ up and $3\cdot \frac{8}{5}$ over from the point $(7,3)$ to reach our maximum. This means the maximum value of $3x + 4y$ occurs at $\left(7 +3\cdot \frac{8}{5}, 3 + 4\cdot \frac{8}{5}\right)$, which is $\left(\frac{59}{5}, \frac{47}{5}\right).$

Plug in those values for $x$ and $y$, and you get the maximum value of $3x + 4y = 3\cdot\frac{59}{5} + 4\cdot\frac{47}{5} = \boxed{73}$, which is option $\boxed{(B)}$.

Solution 2B

Let the tangent point be $P$, and the tangent line's x-intercept be $Q$. Consider the horizontal line starting from center of circle (O) meeting the tangent line at K. Now triangle $OPK$ is 3-4-5, $OP=8$, so $OK = \frac{5}{3}*8 = \frac{40}{3}$. Note that the horizontal distance from $O$ to the origin is $7$, and the horizontal distance from K to Q is 4, ($\frac{4}{3}$ of its y coordinate), so the x-intercept is $7+4+OK = 73/3$. The value of $3x+4y$ is 73 at point $Q$. Note that this value is constant on the tangent line, so there is no need to calculate the coordinate of $P$. $\boxed{(B)}$.

Solution 3

Let $z = 3x + 4y$. Solving for $y$, we get $y = (z - 3x)/4$. Substituting into the given equation, we get \[x^2 + \left( \frac{z - 3x}{4} \right)^2 = 14x + 6 \cdot \frac{z - 3x}{4} + 6,\] which simplifies to \[25x^2 - (6z + 152)x + (z^2 - 24z - 96) = 0.\]

This quadratic equation has real roots in $x$ if and only if its discriminant is nonnegative, so \[(6z + 152)^2 - 4 \cdot 25 \cdot (z^2 - 24z - 96) \ge 0,\] which simplifies to \[-64z^2 + 4224z + 32704 \ge 0,\] which can be factored as \[-64(z + 7)(z - 73) \ge 0.\] The largest value of $z$ that satisfies this inequality is $\boxed{73}$, which is $\boxed{(B)}$.


Solution 4 (Using Answer Choice + Calculus)

Implicitly differentiating the given equation with respect to $x$ yields:

$2x + 2y\frac{dy}{dx} = 14 + 6\frac{dy}{dx}$

Now solve for $\frac{dy}{dx}$ to obtain:

$\frac{dy}{dx} = -\frac{x - 7}{y - 3}$

Set the equation equal to zero to find the maximum occurs at $x = 7$

Plug this back into the equation that we are trying to maximize and see that we are left with: $21 + 4y$.

The only answer choice that can be obtained from this equation is $\bf{73}$

Solution 5 (Lagrange Multipliers)

First, we move all the non-constant terms of the constraint to one side and assign it to the function $g(x,y)$: \[g(x,y)=x^2+y^2-14x-6y.\] Since we are trying to maximize $f(x,y)=3x+4y$, we need to solve for $x$ and $y$ in the system \[\begin{cases}x^2+y^2-14x-6y=6,\\\nabla g(x,y)=\lambda\nabla f(x,y).\end{cases}\] We have that \begin{align*}\nabla f(x,y)&=\begin{pmatrix}\dfrac{\partial}{\partial x}3x+4y\\\dfrac{\partial}{\partial y}3x+4y\end{pmatrix}\\&=\begin{pmatrix}3\\4\end{pmatrix},\end{align*} and \begin{align*}\nabla g(x,y)&=\begin{pmatrix}\dfrac{\partial}{\partial x}x^2+y^2-14x-6y\\\dfrac{\partial}{\partial y}x^2+y^2-14x-6y\end{pmatrix}\\&=\begin{pmatrix}2x-14\\2y-6\end{pmatrix}\end{align*}. To solve the original system, we can solve for $x$ and $y$ in terms of $\lambda$ using our equations from the gradients, then substitute them into the first equation. We have that $x=\frac{3}{2}\lambda+7$ and $y=2\lambda+3$. Substituting into the first equation, we have that \begin{align*}\left(\frac{3}{2}\lambda+7\right)^2+(2\lambda+3)^2-14\left(\frac{3}{2}\lambda+7\right)-6(2\lambda+3)&=6\\\frac{25}{4}\lambda^2&=64\\\lambda&=\pm\frac{16}{5}\end{align*} Using the solutions of $x$ and $y$ in terms of $\lambda$ that we found earlier, we have that \[3x+4y=\frac{25}{2}\lambda+33.\] Because we are trying to maximize this function, we will use the positive solution for $\lambda$. Therefore, after substituting, we have that the largest value of $g(x,y)$ that satisfies $f(x,y)=6$ is $\boxed{\textbf{(B) }73}$.

~qianqian07

Solution 6 (polar coordinates)

Completing the square gives the equation of a circle, $(x-7)^2+(y-3)^2=64.$ Seeing that we would like to maximize $3x+4y,$ we parameterize the circle using polar coordinates: \begin{align*} x&=7+8\cos\theta&y&=3+8\sin\theta. \end{align*} Then, we have $3x+4y=3(7+8\cos\theta)+4(3+8\sin\theta)=33+24\cos\theta+32\sin\theta.$ Since $a\cos\theta+b\sin\theta\le\sqrt{a^2+b^2},$ the desired answer is $33+\sqrt{24^2+32^2}=33+40=\boxed{73}.$ - Ultroid999OCPN

Solution 6 (Alcumus)

The equation $x^2 + y^2 = 14x + 6y + 6$ can be written\[ (x-7)^2 + (y-3)^2 = 8^2, \]which defines a circle of radius 8 centered at $(7,3)$. If $k$ is a possible value of $3x + 4y$ for $(x,y)$ on the circle, then the line $3x + 4y = k$ must intersect the circle in at least one point. The largest value of $k$ occurs when the line is tangent to the circle, and is therefore perpendicular to the radius at the point of tangency. Because the slope of the tangent line is $-3/4$ the slope of the radius is $\ 4/3$. It follows that the point on the circle that yields the maximum value of $3x + 4y$ is one of the two points of tangency,\[ x = 7 + \frac{3 \cdot 8}{5} = \frac{59}{5}, \hspace{.3in} y = 3 + \frac{4 \cdot 8}{5} = \frac{47}{5}, \]or\[ x = 7 - \frac{3 \cdot 8}{5} = \frac{11}{5}, \hspace{.3in} y = 3 - \frac{4 \cdot 8}{5} = - \frac{17}{5}. \][asy] import olympiad; import geometry; size(150); defaultpen(linewidth(0.8)); draw((0,15)--origin--(15,0)); dot("$(7,3)$",(7,3),S); draw(Circle((7,3),8)); line a = line((0,9),(12,0)); line b = line((0,12),(16,0)); line c = line((0,18.3),(18.3*4/3,0)); draw(a^^b^^c); [/asy]

The first point of tangency gives\[ 3x + 4y = 3 \cdot \frac{59}{5} + 4 \cdot \frac{47}{5} = \frac{177}{5}+\frac{188}{5}= 73, \]and the second one gives\[\, 3x + 4y = \frac{33}{5}-\frac{68}{5} = -7.\]Thus, $\boxed{73}$ is the desired maximum, while $-7$ is the minimum.

See also

1996 AHSME (Problems • Answer Key • Resources)
Preceded by
Problem 24 		Followed by
Problem 26
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