In triangle ABC, angles A, B, C form an arithmetic sequence. If angle A = 21 degrees, what is angle C, in degrees?
a = 21
b = 21 + d
c = 21 + 2d d is the common difference Summed, a + b + c = 180
21 + 21+d + 21 + 2d = 180
d = 39
so angle c = 21 + 2d = 99o
