Geometry

Question Analysis:
This question involves triangle angle sum and isosceles triangle properties.
The main focus is on calculating base angles given the vertex angle.

Key Concept Explanation:
Triangle angle sum: 180∘
Base angles of an isosceles triangle are equal.

Step-by-Step Solution:
Given ∠A = 40°, find base angles ∠B and ∠C.
Use the triangle sum rule:
∠A + ∠B + ∠C = 180∘
40∘ + 2x = 180∘
2x = 140∘
x = 70∘
Answer: 70°.

Common Mistakes:
Forgetting that the two base angles are equal.
Not subtracting the vertex angle correctly.

Summary:...