`a)y/[x^2-xy]+x/[y^2-xy]=y/[x(x-y)]-x/[y(x-y)]`
`=[y^2-x^2]/[xy(x-y)]=[-(x-y)(x+y)]/[xy(x-y)]=[-x-y]/[xy]`
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`b)1/[x^2+xy]+2/[y^2-x^2]+1/[xy-x^2]`
`=1/[x(x+y)]+2/[(y-x)(x+y)]+1/[x(y-x)]`
`=[y-x+2x+x+y]/[x(y-x)(x+y)]`
`=[2x+2y]/[x(y-x)(x+y)]=[2(x+y)]/[x(y-x)(x+y)]=2/[x(y-x)]=2/[xy-x^2]`