一些分式数学题
(1)(xy/z)3*(-xz/y)3/(-yz/x)4
(2)x2+2x+1/x2÷25-x2/x2+5x
(3)-4x2+12xy-9y2/4x2-9y2,其中x=3y
(4)已知x/2=y/3=z/4,求xy+yz+zx/x2+y2+z2的值