因式分解
a四次方-1/2a²x²y²+1/16x四次方y四次方
a三次方-a²+1/4a
(3a²+2a-8)²-(a²-2a-8)²
4(x+y)²+25-20(x+y)
(x²+y²)²-4x²y²
a²b²-8abc+16c²