# Need Some Guidance

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• Aug 24th 2009, 05:20 PM
Just4
Need Some Guidance
Solve.
15x^4 - 28x^2 + 5 = 0
substitution should result in a quadratic equation in the variable u, ie

I let u = x^2

15u^4 - 28x^2 + 5= 0
15u^2 - 28u + 5 = 0

I used the quadratic formula

-(-28) + or - sqrt -28^2 - 4(15)(5) / 2(15)
28 + or - sqrt 784+300/30
28 + or - sqrt 1084/30

how'd I do?
• Aug 24th 2009, 05:37 PM
skeeter
Quote:

Originally Posted by Just4
Solve.
15x^4 - 28x^2 + 5 = 0
substitution should result in a quadratic equation in the variable u, ie

I let u = x^2

15u^4 - 28x^2 + 5= 0
15u^2 - 28u + 5 = 0

I used the quadratic formula

-(-28) + or - sqrt -28^2 - 4(15)(5) / 2(15)
28 + or - sqrt 784+300/30
28 + or - sqrt 1084/30

how'd I do?

made a mistake and didn't finish ...

$\displaystyle u = \frac{-28 \pm \sqrt{(-28)^2 - 4(15)(5)}}{2(15)}$

$\displaystyle u = \frac{-28 \pm \sqrt{784 - 300}}{30}$

$\displaystyle u = \frac{-28 \pm \sqrt{484}}{30}$

$\displaystyle u = \frac{-14 \pm 11}{15}$

you're solving for x, remember?