# Math Help - Need Some Guidance

1. ## 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

-(-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?

2. 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

-(-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 ...

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

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

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

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

you're solving for x, remember?