Question 1143013
(A recommendation, next time say in what terms you want the answer to be in)

Answer: {{{(y^2)/(x^2)*cos^2(b)}}}
If {{{a=b}}} and {{{c=d}}} then {{{c/a=d/b}}} so,
{{{sin(a)/tan(a)}}}={{{y*sin(b)/(x*tan(b))}}}

Using the property {{{sin(x)/tan(x)=cos(x)}}} (If you don't understand this notice {{{tanx=sinx/cosx}}})

Which makes the beforementioned equation {{{cos a=(y/x)cos b)}}}
squaring both sides will get the desired equation