2021 Fall AMC 12B Problems/Problem 8

Problem

The product of the lengths of the two congruent sides of an obtuse isosceles triangle is equal to the product of the base and twice the triangle's height to the base. What is the measure, in degrees, of the vertex angle of this triangle?

$\textbf{(A)} \: 105 \qquad\textbf{(B)} \: 120 \qquad\textbf{(C)} \: 135 \qquad\textbf{(D)} \: 150 \qquad\textbf{(E)} \: 165$

Solution 1 (Area)

Let the lengths of the two congruent sides of the triangle be $x$ , then the product desired is $x^2$ .

Notice that the product of the base and twice the height is $4$ times the area of the triangle.

Set the vertex angle to be $a$ , we derive the equation:

$x^2=4(\frac{1}{2}x^2\sin(a))$

$\sin(a)=\frac{1}{2}$

As the triangle is obtuse, $a=150^\circ$ only. We get $\boxed{\textbf{(D)} \ 150}.$

~Wilhelm Z

2021 Fall AMC 12A (Problems • Answer Key • Resources)
Preceded by Problem 16	Followed by Problem 18
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All AMC 12 Problems and Solutions

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2021 Fall AMC 12B Problems/Problem 8

Problem

Solution 1 (Area)