s(teta) = h sin (90-teta)/ sin(teta)

where h is height of the pole
a) Express s(teta) as a single trigonometric expression.
b) At what angle teta will give the shortest shadow of the pole? Longest shadow?
c) In triangle ABC, prove that: tan A + tan B + tan C = tan A tan B tan C

I don't know how do it. Pleaase help thanks