1、=y²/16x的6次方 ×4x的4次方/y²
=1/4x²
2、=(2a-b)/(a²-9b²)-5b/(a²-9b²)
=(2a-6b)/(a²-9b²)
=2(a-3b)/(a+3b)(a-3b)
=2/(a+3b)
3、=(a-2)/a(a+2) ×(a+2)(a-2)/(a-2)²
=1/a