Example1. If `x prop y,` then prove that `x^3+y^3 prop x^2y-xy^2`
`x prop y`
`=> x=M*y` (where constant `M != 0`)
Now `(x^3+y^3) / (x^2y-xy^2)`
`= (M^3y^3+y^3) / (M^2y^3-My^3)`
`= (y^3(M^3+1)) / (My^3(M-1))`
`= ((M^3+1)) / (M(M-1))`
`=` non-zero constant
`:. x^3+y^3 prop x^2y-xy^2`
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