已知实数a不等于b,比较a的4次方—b的4次方与4a的三次方乘以（a—b）的大小

a^4-b^4-4a^3(a-b)
=(a^2+b^2)(a+b)(a-b)-4a^3(a-b)
=(a-b)(b^3+a^2b+ab^2-3a^3)
=(a-b)(a-b)(-3a^2-2ab-b^2)
=-(a-b)^2×(3a^2+2ab+b^2)
=-(a-b)^2×[(a+b)^2+2a^2]
因为a≠b
所以-(a-b)^20
所以a^4-b^4-4a^3(a-b)=-(a-b)^2×[(a+b)^2+2a^2]