费费数学6－2

rosemarys

为什么答案是B？我选D

2. if for all x, x^2+ax+b=(x+c)^2, then a=?

(1)b=0,

(2)c=-3

my answer is:

if x^2+ax+b=(x+c)^2

then x^2+ax+b=x^2+2cx+c^2

so a=2c and b=c^2

(1) b=0 so c=0 so that a=2c=0

(2) a=2c=-6

所以我选d 可是答案是b

smilinglan

A is not right, if a=b=c=o,X can only be 0.

daoying

把a=b=c带入得到等式为X^2=X^2也可为任何数啊.我也认为是D