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5-9.

Multiple Choice: In the triangle below, must be:  

  1. none of these

A triangle with a 42 degree angle between two equal sides. Angle x is opposite one of the equal sides.

The triangle is isosceles meaning sides are equal.

The measure of 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 , then .
So the answer is B.