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Difference between revisions of "1983 AIME Problems/Problem 5"

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Suppose that the sum of the squares of two complex numbers <math>x</math> and <math>y</math> is <math>7</math> and the sum of the cubes is <math>10</math>. What is the largest real value that <math>x + y</math> can have?
 
Suppose that the sum of the squares of two complex numbers <math>x</math> and <math>y</math> is <math>7</math> and the sum of the cubes is <math>10</math>. What is the largest real value that <math>x + y</math> can have?
  
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== Solution ==
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== Solutions ==
 
=== Solution 1 ===
 
=== Solution 1 ===
 
One way to solve this problem is by [[substitution]]. We have
 
One way to solve this problem is by [[substitution]]. We have
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Substituting <math>c=\frac{b^2-7}{2}</math>, we deduce that <math>b^3-21b-20=0</math>, whose roots are <math>-4</math>, <math>-1</math>, and <math>5</math>.
 
Substituting <math>c=\frac{b^2-7}{2}</math>, we deduce that <math>b^3-21b-20=0</math>, whose roots are <math>-4</math>, <math>-1</math>, and <math>5</math>.
 
Since <math>-b</math> is the sum of the roots and is maximized when <math>b=-4</math>, the answer is <math>-(-4)=\boxed{004}</math>.
 
Since <math>-b</math> is the sum of the roots and is maximized when <math>b=-4</math>, the answer is <math>-(-4)=\boxed{004}</math>.
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===Solution 4===
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<math>x^3 + y^3 = 10 = (x+y)(x^2-xy+y^2) = (x+y)(7-xy) \implies xy = 7 - \frac{10}{x+y}.</math>  Also, <math>(x+y)^3 = x^3 + 3x^2y+3xy^2+y^3 = 10 + 3xy(x+y).</math>  Substituting our above into this, we get <math>10 + 3(\frac{10}{x+y})(x+y) = 21x+21y-20 = (x+y)^3</math>.  Letting <math>p = x+y</math>, we have that <math>p^3 - 21p + 20 = 0</math>.  Testing <math>p = 1</math>, we find that this is a root, to get <math>(p-1)(x^2+5x-20) = 0 \implies p = -5, 1, 4 \implies \boxed{4}</math>
  
 
== See Also ==
 
== See Also ==
 
{{AIME box|year=1983|num-b=4|num-a=6}}
 
{{AIME box|year=1983|num-b=4|num-a=6}}
 
[[Category:Intermediate Algebra Problems]]
 
[[Category:Intermediate Algebra Problems]]

Revision as of 13:24, 8 December 2021

Problem

Suppose that the sum of the squares of two complex numbers $x$ and $y$ is $7$ and the sum of the cubes is $10$. What is the largest real value that $x + y$ can have?

Solutions

Solution 1

One way to solve this problem is by substitution. We have

$x^2+y^2=(x+y)^2-2xy=7$ and $x^3+y^3=(x+y)(x^2-xy+y^2)=(7-xy)(x+y)=10$

Hence observe that we can write $w=x+y$ and $z=xy$.

This reduces the equations to $w^2-2z=7$ and $w(7-z)=10$.

Because we want the largest possible $w$, let's find an expression for $z$ in terms of $w$.

$w^2-7=2z \implies z=\frac{w^2-7}{2}$.

Substituting, $w^3-21w+20=0$, which factorizes as $(w-1)(w+5)(w-4)=0$ (the Rational Root Theorem may be used here, along with synthetic division).

The largest possible solution is therefore $x+y=w=\boxed{004}$.

Solution 2

An alternate way to solve this is to let $x=a+bi$ and $y=c+di$.

Because we are looking for a value of $x+y$ that is real, we know that $d=-b$, and thus $y=c-bi$.

Expanding $x^2+y^2=7+0i$ will give two equations, since the real and imaginary parts must match up.

$(a+bi)^2+(c-bi)^2=7+0i$

$(a^2+c^2-2b^2)+(2ab-2cb)i=7+0i$

Looking at the imaginary part of that equation, $2ab-2cb=0$, so $a=c$, and $x$ and $y$ are actually complex conjugates.

Looking at the real part of the equation and plugging in $a=c$, $2a^2-2b^2=7$, or $2b^2=2a^2-7$.

Now, evaluating the real part of $(a+bi)^3+(a-bi)^3$, which equals $10$ (ignoring the odd powers of $i$, since they would not result in something in the form of $10+0i$):

$a^3+3a(bi)^2+a^3+3a(-bi)^2=10$

$2a^3-6ab^2=10$

Since we know that $2b^2=2a^2-7$, it can be plugged in for $b^2$ in the above equation to yield:

$2a^3-3a(2a^2-7)=10$

$-4a^3+21a=10$

$4a^3-21a+10=0$

Since the problem is looking for $x+y=2a$ to be a positive integer, only positive half-integers (and whole-integers) need to be tested. From the Rational Roots theorem, $a=10, a=5, a=\frac{5}{2}$ all fail, but $a=2$ does work. Thus, the real part of both numbers is $2$, and their sum is $\boxed{004}$.

Solution 3

Begin by assuming that $x$ and $y$ are roots of some polynomial of the form $w^2+bw+c$, such that by Vieta's Formulæ and some algebra (left as an exercise to the reader), $b^2-2c=7$ and $3bc-b^3=10$. Substituting $c=\frac{b^2-7}{2}$, we deduce that $b^3-21b-20=0$, whose roots are $-4$, $-1$, and $5$. Since $-b$ is the sum of the roots and is maximized when $b=-4$, the answer is $-(-4)=\boxed{004}$.

Solution 4

$x^3 + y^3 = 10 = (x+y)(x^2-xy+y^2) = (x+y)(7-xy) \implies xy = 7 - \frac{10}{x+y}.$ Also, $(x+y)^3 = x^3 + 3x^2y+3xy^2+y^3 = 10 + 3xy(x+y).$ Substituting our above into this, we get $10 + 3(\frac{10}{x+y})(x+y) = 21x+21y-20 = (x+y)^3$. Letting $p = x+y$, we have that $p^3 - 21p + 20 = 0$. Testing $p = 1$, we find that this is a root, to get $(p-1)(x^2+5x-20) = 0 \implies p = -5, 1, 4 \implies \boxed{4}$

See Also

1983 AIME (Problems • Answer Key • Resources)
Preceded by
Problem 4 		Followed by
Problem 6
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All AIME Problems and Solutions
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