### Home > GC > Chapter 5 > Lesson 5.3.4 > Problem 5-105

5-105.

**Multiple Choice:**In the triangle at right,*x*must be:Homework Help ✎42°

69°

21°

138°

none of these

The triangle is isosceles meaning 2 sides are equal.

The measure of *x* must be the same as the other unknown angle because base angles are equal in an isosceles triangle.

Since the sum of all the interior angles of a triangle must be 180°, then *x* + *x* + 42° = 180°.

So the answer is (b).