求助：天山－7－5

mindfrees

if b,c,d are constants and x²+bx+c=(x+d)², for all values of x, what is the value of c?

(1)d=3

(2)b=6

答案是D

我选的A, 当x＝0时，c=d²=9

不知道为什么单独知道b的值可以求出c

请大家给我点帮助吧

huahua115

b=6 means d=6 too due to x^2+bx+c=x^2+dx+d^2

we can conclude that d=b,so we need to find out what is the number

about b or d

so the information which tell us that b=6 can sovle the problem