# Thread: roots of cubic equations

1. ## roots of cubic equations

the roots of the cubic equation
x^3 - 15x^2 + px - 105 = 0

are 5-k, 5 and 5+k
Find the values of the constants p and k

i dont know what to do or how to start please some one help!!

2. ## Re: roots of cubic equations

Originally Posted by mathkid12
the roots of the cubic equation
x^3 - 15x^2 + px - 105 = 0

are 5-k, 5 and 5+k
Find the values of the constants p and k

i dont know what to do or how to start please some one help!!
perform synthetic division with a 0 remainder

3. ## Re: roots of cubic equations

sorry to say this but what!!!!!! synthetic devision >??

4. ## Re: roots of cubic equations

might want to look it up ... be sure to spell it right

5. ## Re: roots of cubic equations

Originally Posted by mathkid12
the roots of the cubic equation
x^3 - 15x^2 + px - 105 = 0

are 5-k, 5 and 5+k
Find the values of the constants p and k

i dont know what to do or how to start please some one help!!
5 is a root so sub 5 into the equation and you will get p.
In a cubic equation starting with x^3 the product of the roots is the constant term, in this case -105 So 5(5-k)(5+k)= -105