(a+1)(a-2)(a-3)(a-6)= [(a+1) (a-6)][ (a-2)(a-3)]
=(a^2-5a-6)(a^2-5a+6)
=(a^2-5a)^2-36
=a^4-10a^3+25a^2-36
(a+b)^3=(a+b)(a^2+2ab+b^2)
=a^3+2a^2b+ab^2+ a^2 b +2ab^2+b^3
=a^3+b^3+3ab(a+b)
(a+b)^4=(a+b) (a+b)^3
=(a+b)[ a^3+b^3+3ab(a+b)]
=a^4+b^4+4ab(a^2+b^2)
好烦啊