1) Let's prove this identity
Since tan (θ) = sin(θ)/cos((θ)
And cot((θ) = cos ((θ)/sin((θ)
2) Let's plug it into:

Simplifying (dividing) sin(θ) on the numerator, with sin (θ) on the denominator and similarly cos (θ) with cos(θ) we'll get to 1 over 1 time 1 over 1 = 1
Then 1=1