There is a negative 1/sin(a) and a positive 1/sin(a), so we can cancel both of them out.
We have
1/csc(a) - cot(a) = -1/csc(a) + cot(a) remaining.
One is positive, and one is negative, and they are both equal, so we can also cancel each of them out.
We have
0 = 0 left.
0 equals 0, so we are done explaining.
Hope this helped
