# roots of cubic equations

• May 6th 2012, 02:22 PM
mathkid12
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!!
• May 6th 2012, 03:21 PM
skeeter
Re: roots of cubic equations
Quote:

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
• May 6th 2012, 03:29 PM
mathkid12
Re: roots of cubic equations
sorry to say this but what!!!!!! synthetic devision >??
• May 6th 2012, 03:36 PM
skeeter
Re: roots of cubic equations
might want to look it up ... be sure to spell it right
• May 6th 2012, 10:04 PM
biffboy
Re: roots of cubic equations
Quote:

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