数学题(要结果),一.分解因式:xy^3-4xy=?二.计算:（3a/a-3 -a/a+3）*a^2-9/a=?

因式分解:
xy^3-4xy
=xy(y^2-4)
=xy(y-2)(y+2)

计算:
[(3a/(a-3)-a/(a+3)]*(a^2-9)/a
={[(3a(a+3)-a/(a-3)]/[(a-3)(a+3)]}*(a^2-9)/a
={[(3a^2+9a-a^2+3a)]/[(a-3)(a+3)]}*(a^2-9)/a
={(2a^2+6a)/[(a-3)(a+3)]}*(a^2-9)/a
={2a(a+3)/[(a-3)(a+3)]}*(a^2-9)/a
=[2a/(a-3)]*(a^2-9)/a
=[2a/(a-3)]*[(a-3)(a+3)]/a
=[2a(a+3)]/a
=2(a+3)
=2a+6

xy^3-4xy
=xy(y^2-4)
=xy(y-2)(y+2)

[3a/(a-3)-a/(a+3)]*[(a^2-9)/a]
=[3a/(a-3)][(a^2-9)/a]-[a/(a+3)][(a^2-9)/a]
=[3a/(a-3)][(a+3)(a-3)/a]-[a/(a+3)][(a+3)(a-3)/a]
=3(a+3)-(a-3)
=3a+9-a+3
=2a+12

xy^3-4xy
=xy(y^2-4)
=xy(y-2)(y+2)