If a2 - b2 = 9 and a*b = 4, find a4 + b4.	Choose the correct answer.
A) 32
B) 90
C) 113
D) 92
E) 81

Rationale: "a 2 - b 2 = 9 : given a 4 + b 4 - 2 a 2 b 2 = 92 : square both sides and expand . a * b = 4 : given a 2 b 2 = 42 : square both sides . a 4 + b 4 - 2 ( 16 ) = 81 : substitute a 4 + b 4 = 113 correct answer c"
Answer: C