If a + b = 11 and ab = 15, then $a^2 + b^2 $ is equal to :

90 91 93 92

91

We know that, a + b = 11 and ab = 15  (a + b)2 = a2 + b2 + 2ab = 112 = a2 + b2 + 2 × 15 = a2 + b2 = 121 - 30 a2 + b2 = 91