2021 Fall AMC 12B Problems/Problem 10

Problem

What is the sum of all possible values of $t$ between $0$ and $360$ such that the triangle in the coordinate plane whose vertices are $(\cos 40^\circ,\sin 40^\circ), (\cos 60^\circ,\sin 60^\circ), \text{ and } (\cos t^\circ,\sin t^\circ)$ is isosceles?

$\textbf{(A)} \: 100 \qquad\textbf{(B)} \: 150 \qquad\textbf{(C)} \: 330 \qquad\textbf{(D)} \: 360 \qquad\textbf{(E)} \: 380$

Solution 2

Denote $A = \left( \cos 40^\circ , \sin 40^\circ \right)$ , $B = \left( \cos 60^\circ , \sin 60^\circ \right)$ , and $C = \left( \cos t^\circ , \sin t^\circ \right)$ .

Case 1: $CA = CB$ .

We have $t = 50$ or $230$ .

Case 2: $BA = BC$ .

We have $t = 80$ .

Case 3: $AB = AC$ .

We have $t = 20$ .

Therefore, the answer is $\boxed{\textbf{(E) }380}$ .

~Steven Chen (www.professorchenedu.com)

2021 Fall AMC 12B (Problems • Answer Key • Resources)
Preceded by Problem 9	Followed by Problem 11
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All AMC 12 Problems and Solutions

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2021 Fall AMC 12B Problems/Problem 10

Problem

Solution 2