If a2 - b2 = 9 and a*b = 4, find a4 + b4.
Choose the correct choice
A) 32	B) 90	C) 113	D) 92	E) 81
a2 - b2 = 9 : given   a4 + b4 - 2a2b2 = 92 : square both sides and expand.   a*b = 4 : given   a2b2 = 42 : square both sides.   a4 + b4 - 2(16) = 81 : substitute   a4 + b4 = 113   correct answer 113
<result>C</result>
If a2 - b2 = 9 and a*b = 4, find a4 + b4.