algebra
posted by anon .
solve the following in the real number system
x^3 + 8x^2 + 10x  15 = 0
Please help me!

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+723015=0
so, divide (x+3) into x^3+8x^2+10x15 and get the quadratic. Then solve it with the quadratic equation.
I get x^2+5x5 check that. 
Thank you I think I understand it better with Bob's answer.