can someone help explain this step to me?

I have

$$\left(\frac{E_y}{b}-\frac{E_x}{a}\cos\delta\right)^2=\left(1-\frac{E_x^2}{a^2}\right)\sin^2\delta$$and apparantly that is equivalent to$$\frac{E_y^2}{b^2}+\frac{E_x^2}{a^2}-2\frac{E_xE_y}{ab}\cos{\delta}=\sin^2{\delta}$$

Question

can someone help explain this step to me?

I have

$$\left(\frac{E_y}{b}-\frac{E_x}{a}\cos\delta\right)^2=\left(1-\frac{E_x^2}{a^2}\right)\sin^2\delta$$and apparantly that is equivalent to$$\frac{E_y^2}{b^2}+\frac{E_x^2}{a^2}-2\frac{E_xE_y}{ab}\cos{\delta}=\sin^2{\delta}$$

richyw · Answer

when I expand out the left side though I get $$\frac{x^2 \cos ^2(d)}{a^2}-\frac{2 x y \cos (d)}{a b}+\frac{y^2}{b^2}$$(I just made $E_x=x,\; E_y=y$ anad $\delta=d$ to save time

richyw · Answer

so my problem is how the $\cos^2{d}$ gets out of that first term.

richyw · Answer

aha . nevermind I got it.