先把他拆开为X^3-11X^2+10X+21X-21然后前三项提一个X,得X(X-11X+10)+21（X-1）然后前边在变形X(X-10)(X-1)+21(X-1)提一个（X-1）得（X-1）(X^2-10X+21)最后的（X-1）(X-3)(X-7)然后OK!