If a2 - b2 = 5 and a*b = 2, find a4 + b4.
Choices.
A) 50
B) 72
C) 25
D) 92
E) 33
a2 - b2 = 5 : given   a4 + b4 - 2a2b2 = 52 : square both sides and expand.   a*b = 2 : given   a2b2 = 22 : square both sides.   a4 + b4 - 2(4) = 25 : substitute   a4 + b4 = 33   correct answer 33
<result>E</result>
If a2 - b2 = 5 and a*b = 2, find a4 + b4.