Difference between revisions of "2015 AMC 10A Problems/Problem 17"

Revision as of 14:02, 8 October 2023

Problem

A line that passes through the origin intersects both the line $x = 1$ and the line $y=1+ \frac{\sqrt{3}}{3} x$ . The three lines create an equilateral triangle. What is the perimeter of the triangle?

$\textbf{(A)}\ 2\sqrt{6} \qquad\textbf{(B)} \ 2+2\sqrt{3} \qquad\textbf{(C)} \ 6 \qquad\textbf{(D)} \ 3 + 2\sqrt{3} \qquad\textbf{(E)} \ 6 + \frac{\sqrt{3}}{3}$

Solution 1

Since the triangle is equilateral and one of the sides is a vertical line, the triangle must have a horizontal line of symmetry, and therefore the other two sides will have opposite slopes. The slope of the other given line is $\frac{\sqrt{3}}{3}$ (can be find out easily by using 60 degree angle of the triangle) so the third must be $-\frac{\sqrt{3}}{3}$ . Since this third line passes through the origin, its equation is simply $y = -\frac{\sqrt{3}}{3}x$ . To find two vertices of the triangle, plug in $x=1$ to both the other equations.

$y = -\frac{\sqrt{3}}{3}$

$y = 1 + \frac{\sqrt{3}}{3}$

We now have the coordinates of two vertices, $\left(1, -\frac{\sqrt{3}}{3}\right)$ and $\left(1, 1 + \frac{\sqrt{3}}{3}\right)$ . The length of one side is the distance between the y-coordinates, or $1 + \frac{2\sqrt{3}}{3}$ .

The perimeter of the triangle is thus $3\left(1 + \frac{2\sqrt{3}}{3}\right)$ , so the answer is $\boxed{\textbf{(D) }3 + 2\sqrt{3}}$

Solution 2

Draw a line from the y-intercept of the equation $y=1+ \frac{\sqrt{3}}{3} x$ perpendicular to the line $x=1$ . There is a square of side length 1 inscribed in the equilateral triangle. The problem becomes reduced to finding the perimeter of an equilateral triangle with a square of side length 1 inscribed in it. The side length is $2\left(\frac{1}{\sqrt{3}}\right) + 1$ . After multiplying the side length by 3 and rationalizing, you get $\boxed{\textbf{(D) }3 + 2\sqrt{3}}$ .

Solution 3 (Intuitive)

We have $y = mx.$

With the first condition, we have that $y=m.$

Then, we have $1 + \frac{\sqrt{3}}{3} x = mx$

Dividing both sides by $x$ on the second and putting over a common denominator gets us $\frac{3 + x \sqrt{3}}{3x} = m.$ The only answer in the answer choices that satisfies this is (D)

Video Solution

https://youtu.be/-l1Kawq_hds

~savannahsolver

Revision as of 06:36, 4 June 2023 (view source) Harshsahu (talk \| contribs) (→‎Solution 1) ← Older edit		Revision as of 14:02, 8 October 2023 (view source) Sonje (talk \| contribs) m (→‎See Also) Newer edit →
Line 41:		Line 41:

	{{AMC10 box\|year=2015\|ab=A\|num-b=16\|num-a=18}}		{{AMC10 box\|year=2015\|ab=A\|num-b=16\|num-a=18}}
−	{{MAA Notice}}a	+	{{MAA Notice}}

2015 AMC 10A (Problems • Answer Key • Resources)
Preceded by Problem 16	Followed by Problem 18
1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15 • 16 • 17 • 18 • 19 • 20 • 21 • 22 • 23 • 24 • 25
All AMC 10 Problems and Solutions

Page

Toolbox

Search