`#DyHungg`
`a) xy(x^5-y^3)-x^2y(x^4-y^3)`
`=x^6y-xy^4-x^6y+x^2y^4`
`=-xy^4+x^2y^4`
........
`x^3y^4(x^2-y^3)-x^2y^3(x^4-y^4)`
`=x^5y^4-x^3y^7-x^6y^3+x^2y^7`
...................
`90.10^n-10^(n+2)+10^(n+1)`
`=900^n-10^(n+2)+10^(n+1)`
.................
`y(x^(n-1)+y^(n-1))-x^(n-1)(x+y)`
`=x^(n-1)y+y^(n-1+1)-x^(n-1+1)-x^(n-1)y`
`=y^n-x^n`