[求助]10月math机经#353 请指点迷惑

zekeo · 发表于 2003-10-27 16:09:00

353) (X+A)(X+B)=X^2+3X+C, 求C
1）A=1
2）B=2
选D
答案: D
思路:

Could the answer be c?

(X+A)(X+B)=X^2+3x+C ==> (A+B)X+AB = 3X+C
Can we assume A+B must be equal to 3 ? Should we consider the case when X=0,
then we need both A and B to get the value of C.

谢谢!

lixiaoyang · 发表于 2003-10-27 16:22:00

(a+b)=3
ab=c
A=1 =>B=2=>C=2
B=2=>A=1=>C=2.
not "can we assume" but we must assume a+b=3 and ab=c

[求助]10月math机经#353 请指点迷惑

[求助]10月math机经#353 请指点迷惑

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