Solving $x^{2} = 5 x + 7$

By Jack Sutcliffe (P2879) on Thursday, September 7, 2000 - 08:52 pm :

Does anybody know HOW to do this equation as it would really help me a lot:

Solve the equation x² =5x+7 giving your answers correct to 3 sig. fig.

Thankyou for your help, this is probably easy for you but I just can't quite do it.

Cheers,
Jack.

P.S. I am 15 years old.

By Marion Palles-Clark (Mhp21) on Thursday, September 7, 2000 - 10:47 pm :

Hi Jack!

Do you remember being told the way to solve quadratic equations that you can't factorise?

Well, there is an equation as follows:

If

$begin{displaymath}ax^2 + bx + c = 0 end{displaymath}$

then

$begin{displaymath}x = frac{-b pm sqrt{b^2-4ac}}{2a} end{displaymath}$

So if you re-write your original equation in the form ax² + bx + c = 0 (so you'll have x² - 5x - 7 = 0) you should be able to work it out.

Don't forget you'll end up with 2 solutions!

Marion