Answer:
y = 90°
Explanation:
You want the value of y in the diagram showing y as the angle at the base where the isosceles triangle a.pex angle bisector meets the base.
Angle sum theorem
The angle sum theorem tells you the sum of angles in a triangle is 180°.
Consider the large outside triangle. The sum of its angles is ...
60° +2x +60° = 180°
2x = 60° . . . . . . . . . . . . subtract 120°
x = 30° . . . . . . . . . . divide by 2
Now, consider the left smaller triangle. The sum of its angles is ...
60° +x +y = 180°
60° +30° +y = 180° . . . . . . use the known value of x
y = 90° . . . . . . . . . . . . subtract 90°
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Additional comment
The a.pex angle bisector of an isosceles triangle is always an altitude that meets the base at right angles. The meeting point is the midpoint of the base, so that segment is also a median.
The halves of the isosceles triangle are congruent right triangles, so the base angle is complementary to the angle marked x.