3(x+y)²*[-3(x+y)]²
=3(x+y)²*9(x+y)²
=27(x+y)^4
（X²+4²）-16X²
=(X²-4X+16)(X²+4X+16)
10a³-8a²
=2a²(5a-4)
a²+2ab+b²-1
=(a+b)²-1²
=(a+b+1)(a+b-1)
a³-6a²+9a
=a(a²-6a+9)
=a(a-3)²
已知a(a-1)-(a²-b)=2
a²-a-a²+b=2
b-a=2
求：a²+b²/2-ab
=(b²-2ab+a²)/2
=(b-a)²/2
=4/2
=2
先化简再求值：
[（xy+2）(xy-2)-2(x²y²-2)]÷（xy）
=(x²y²-4-2x²y²+4)÷（xy）
=(-x²y²)/(xy)
=-xy
=-10×(-1/25)
=2/5