1.原式=ab(a^2-2ab+b^2)=ab(a-b)^2
把a-b=5,ab=3代入
那么原式=3*5^2=75
2．­原式=(3a-7-a-5)(3a-7+a+5)/(4(a-6))
=(2a-12)(4a-2)/(4(a-6))
=4(a-6)(2a-1)/(4(a-6))
=2a-1
当a=1/50时
原式=(1/25)-1
=-0.75
3.令x^2-2x=t
原式=t(t+2)+1
=t^2+2t+1
=(t+1)^2
=(x^2-2x+1)^2
=(x-1)^4