用n^5表示n的5次方.
n^5-6n^3+9n (先提出公因子n)
=n(n^4-6n^2+9)
=n[(n^4-3n^2)-3(n^2-3)] (中括号两个式子中提出公因子n^2-3)
=n(n^2-3)*(n^2-3)
=n(n^2-3)^2