Posted by Emily on Tuesday, December 8, 2009 at 11:56pm.
Solve the equation x^3  2x + 4 = 0.
==> I know the answer is 2, but I can't seem to remember how to solve these by hand without using a graphing calculator. Any help is greatly appreciated!! :)

Precalculus  Reiny, Wednesday, December 9, 2009 at 12:17am
Unless they factor, cubic equations cannot be easily solved "by hand"
In this case, try subbing ±1 and ±2 into the equation to get a zero.
You were right to find that x = 2 works
so (x+2) must be a factor.
Use long algebraic division or synthetic division to find the other factor to be (x^2  2x + 2)
using the quadratic formula on
x^2  2x + 2 = 0 gives me two complex roots, namely (1 ± i)/2

correction  Precalculus  Reiny, Wednesday, December 9, 2009 at 12:23am
the two complex roots should have been just
1 ± i

Precalculus  Emily, Wednesday, December 9, 2009 at 12:32am
Ohhh I see. Thanks Reiny this helped a lot!! :)
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