已知x-y除以x+y=3,求代数式x+y分之2（x-y)-3(x-y)分之x+y的值.

解原式=2(x-y)/(x+y)-(x+y)/3(x-y)
=6(x-y)(x-y)/[3(x+y)(x-y)]-(x+y)x+y)/[3(x-y)(x+y)]
=5(x-y)(x-y)/[(x+y)(x-y)]
=5(x-y)/(x+y)
=5×3=15