/sci/ - Science & Math

I'm struggling to prove this identity following my professor's instructions.

Here's what i've done so far.

Sin(alpha), gives us that BD=Sin(alpha)
Cos(alpha), gives us that AD=Cos(alpha)

From these results,

Cos(Beta), gives us that AC=Cos(alpha)/ Cos(Beta)

Sin(Beta) gives us that Sin(Beta)*Cos(alpha)/Cos(Beta)=CD

But i'm not sure how to find the area of the triangle in two ways to get our desired result.

Am I even on the right track?