hi all,

could some please help with the following question.

The remainder when $\displaystyle 2x^3 + ax^2 + 4$ is dividied by $\displaystyle x - 2$ is five times more that when it is divided by $\displaystyle x - 1$.

Find a.

As far as i can see ( i remember to set x to a value that makes divisor 0 since dividend=divisor x quotient + remainder) :

$\displaystyle f(x)=2x^3+ax^2+4$

$\displaystyle f(1)=2+a+4$

$\displaystyle f(2)=16+4a+4$

and

$\displaystyle f(2)=5f(1)$

$\displaystyle 20+4a = 5(6+a)$

$\displaystyle 20+4a=30+5a$

$\displaystyle -10=a$

must be doing something stupid because the book says the answer is -3

thanks sammy