How do you determine the x-intercepts of the polynomial function y = -(2x+5)^3 - 20

I did this:

0 = -(2x+5)^3 - 20

20 = -2x+5)^3

-5 3sqrt(20/-2) (3 before the sqrt being the little 3)

x= 10.7

The answer should be -3.86 however.

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- Mar 8th 2011, 11:08 AMDevi09Determining x-intercepts of polynomial function
How do you determine the x-intercepts of the polynomial function y = -(2x+5)^3 - 20

I did this:

0 = -(2x+5)^3 - 20

20 = -2x+5)^3

-5 3sqrt(20/-2) (3 before the sqrt being the little 3)

x= 10.7

The answer should be -3.86 however. - Mar 8th 2011, 11:28 AMmasters
- Mar 9th 2011, 02:34 AMHallsofIvy
You've dropped a "(". It should be 20= -(2x+5)^3. Now "undo" that. -20^{1/3}= 2x+ 5

2x= -20^{1/3}- 5, x= (-20^{1/3})/2

Quote:

-5 3sqrt(20/-2) (3 before the sqrt being the little 3)

But your real error is that you divided by two**before**taking the cuberoot. You must take the cuberoot first because the "2x+ 5" is inside the cube.

Quote:

x= 10.7

The answer should be -3.86 however.