Posted by anon on Friday, December 9, 2011 at 4:36pm.
Since it is hard to factor a general cubic, look for easy roots. They will be among ±1,3,5,15
A little synthetic division shows that we have
(x+3)(x^2 + 5x - 5)
Now you can use the quadratic formula to get the other two roots:
(-5 ± 3√5)/2
by inspection, x=-3 is a solution.
-27+72-30-15=0
so, divide (x+3) into x^3+8x^2+10x-15 and get the quadratic. Then solve it with the quadratic equation.
I get x^2+5x-5 check that.
Thank you I think I understand it better with Bob's answer.
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