解
x²y-xy²
=xy(x-y)
=5×1
=5
a²b+ab²=1
∴ab(a+b)=1
∵a+b=3
∴3ab=1
∴ab=1/3a(a-b)-a+b=?=a(a-b)-(a-b)
=(a-1)(a-b)