# Thread: Completing the Square and Quadratic Formula..NEED HELP PLEASE

1. ## Completing the Square and Quadratic Formula..NEED HELP PLEASE

Solve the formula for y?
x^2+y^2=r^2
I subrtracted x^2 from both sides and then divided y^2 on both sides...am I on the right track or totally off?

Change the equation to Quadratic Form and then solve using any method?

x(2x+1)/x-4=36/x-4

2. 1. Given $\displaystyle x^2 + y^2 = r^2$:

Subtract $\displaystyle x^2$ from both sides.
$\displaystyle y^2 = r^2 - x^2$

Take the square root of both sides.
$\displaystyle y = \pm\sqrt{r^2 - x^2}$

2. Given $\displaystyle \frac{x(2x + 1)}{x - 4} = \frac{36}{x - 4}$:

Multiply both sides by x - 4.
$\displaystyle x(2x + 1) = 36$

Distribute x and subtract 36 from both sides.
$\displaystyle 2x^2 + x - 36 = 0$

Solve using the quadratic formula.

3. Here are the general rule. If your quadratic equation is in the form $\displaystyle ax^2+bx+c =0$, then for:

Completing the square - $\displaystyle a\left[\left(x + \frac{b}{2a}\right)^2 - \left(\frac{b}{2a}\right)^2 + \frac{c}{a}\right]=0$.

Quadratic equation - $\displaystyle x = \frac{-b \pm \sqrt{b^2-4ac}}{2a}$.