Thread: Finding expression of cubic function

Find an expression for a cubic function if f(1)=6 and f(-1)=f(0)=f(2)=0.

-I understand that one part of the equation could be (x+1)(x-2) since f(-1)=0 and f(2)=0. Now for the gradient, which is where i am having the trouble. The answer comes out to be -3x(x+1)(x-2). I don't know how the -3x was obtained. Do i uses rise over run formula to obtain the -3......such as (1,6) (-1,0).....still by doing this i come to a gradient of 3 not -3.

Originally Posted by johnsy123

Find an expression for a cubic function if f(1)=6 and f(-1)=f(0)=f(2)=0.

-I understand that one part of the equation could be (x+1)(x-2) since f(-1)=0 and f(2)=0. Now for the gradient, which is where i am having the trouble. The answer comes out to be -3x(x+1)(x-2). I don't know how the -3x was obtained. Do i uses rise over run formula to obtain the -3......such as (1,6) (-1,0).....still by doing this i come to a gradient of 3 not -3.

You cubic can be written in the form , where are your x intercepts. You know these are -1, 0 and 2, so you have



Now substitute the last point to evaluate the value of a.