Simplifying the LHS of cot2Acsc2B−cot2Bcsc2A=cot2A−cot2B.
cot2Acsc2B−cot2Bcsc2A=cot2A(1+cot2B)−cot2B(1+cot2A)
=cot2A+cot2Acot2B−cot2B−cot2Acot2B
=cot2A−cot2B
=RHS