The average of a a, b, c is 6 and a-b =4, ab = 21, What is the value of c ?
A
6
B
7
C
8
D
9
Correct Answer: 8
a + b + c = 6 ×3 = 18 ( a + b)2 = ( a - b) 2 + 4ab = 42 + 4 × 21 = 16 + 84 = 100 a + b = 10 10 + c = 18 c = 8