[tex]R.H.S\\\\=(a+b)(a^3-a^2b-ab^2+b^3)\\\\=a^4-a^3b-a^2b^2+ab^3 + ba^3-a^2b^2-ab^3+b^4\\\\=a^4+b^4-2a^2b^2\\\\\neq a^4 +b^4\\\\\\\text{Hence, }(a+b)(a^3-a^2b-ab^2+b^3) =a^4+b^4 ~ \text{is not true.}[/tex]
