Expressing cos 50 using x and y Given that cos15=x and sin35=y, express cos50 in x and y. 
I tried solving it using the addition theorem of trigonometry:
cos50=cos (15+35)=cos15cos35-sin15sin35
                 =xcos35-ysin15
and I am stuck ;(

Some help please? This is my revision for my graduation test. 
P/S The answer given is cos50=x[(1-y^2)^1/2]-y[(1-x^2)^1/2]
$\cos(50)=\cos(35+15)=\cos35\cos15-\sin35\sin15=x\sqrt{1-y^2}-y\sqrt{1-x^2}.$ 

$\text{using}:$

$\sin^235+\cos^235=y^2+\cos^235=1, \text{or} \cos^235=1-y^2.$

Similarly, $\sin^215=1-x^2$
$\cos(\theta) = \sqrt{1-\sin^2(\theta)}$; and so
$$
\cos(50) = x\sqrt{1-y^2} - y\sqrt{1-x^2}
$$