若分解因式x2+mx-21=(x+3)(x+n),则m的值为( ) A.2 B.-2 C.4 D.-4
问题描述:
若分解因式x2+mx-21=(x+3)(x+n),则m的值为( )
A. 2
B. -2
C. 4
D. -4
答
(x+3)(x+n)=x2+nx+3x+3n=x2+(n+3)x+3n,
∵x2+mx-21=(x+3)(x+n),
∴x2+mx-21=x2+(n+3)x+3n,
∴m=n+3,-21=3n,
解得:n=-7,m=-4,
故选D.