Problem: In this diagram AB and AC are the equal sides of an isosceles triangle ABC, in which is inscribed equilateral triangle DEF. Designate angle BFD by a, angle ADE by b​, and angle FEC by c​. Then
Answer Choices:
A. b=2a+c​
B. b=2a−c​
C. a=2b−c​
D. a=2b+c​
E. none of these
Solution:
b+60=a+B
a+60=c+C
∴b−a=a−c+B−C
Since B=C, we have b+c=2 a, a=\dfrac{b+c}
