# Thread: 3D geometry with quadriatic problem need real help!!!

1. ## standard deviation need help!!!!

IM using my brothers account and so Im kinda stuck on these questions thanks

2. Hi there,

Just work through it step by step, each time subbing what you know

So we know that

$\displaystyle L=\frac{1}{3}(E-24x)$

we're told what E is so we can now write.

$\displaystyle L=\frac{1}{3}(98-24x) = \frac{98}{3}-8x$

Now we can put this value of L into our formula for A, which gives us

$\displaystyle 448 = 12x^2 +12\big(\frac{98}{3} - 8x \big)x$
$\displaystyle 448 = 12x^2 + 392x - 96x^2$

Rearrange a bit

$\displaystyle 84x^2 - 392x + 448= 0$

All the terms have a common factor of 28, so we can divide through to get

$\displaystyle 3x^2 - 14x + 16 = 0$

as required

3. ## thanks

that was knida what I was doing in the first place but instead of leaving the 98/3 as a fraction I left it as a decimal so i thnk thats where I went rong... anyway thanks +rep

4. Fractions are much nicer to work with algebraically, so keep everything as a fraction for as long as you can