Hi!
Let's first expand the cotangent of theta.
You'll probably know that tangent will be "y/x", or
, and cotangent is the reciprocal of this, meaning that it is "x/y" or
.
That means that we are now given this equation.

That'll multiply to:

We'll want a common denominator to add by multiplying
to both top and bottom of the second term:


Pythagorean Identity states that
, so substitute that in:

Which simplifies to:

Hope this helps!