If a - b = 5 and a2 + b2 = 31, find the value of ab.
Select the correct option
A) 9  B) 2  C) 3  D) 7  E) 12

Rationale: "2 ab = ( a 2 + b 2 ) - ( a - b ) 2 = 31 - 25 = 6 ab = 3 . answer : c"
Answer: C