If a - b = 8 and ab = 9, then the value of a + b is

±9 ±7 ±8 ±10

±10

a - b = 8 ab = 9 ( a - b )2 = a2 + b2 - 2ab ( 8 )2 = a2 + b2 - 2 × 9 64 = a2 + b2 - 2 × 9 a2 + b2 = 82 a + b = \(\sqrt {82 + 2 × 9}\) a + b = ±10