LHS:
#=sintheta/(1 - costheta/sintheta) + costheta/(1 - sintheta/costheta)#
#=sintheta/((sin theta - costheta)/sintheta) + costheta/((costheta - sin theta)/costheta)#
#=(sinthetasintheta)/(sin theta - costheta) + (costhetacostheta)/(costheta - sintheta)#
#=(-sin^2theta + cos^2theta)/(costheta - sin theta)#
#=(cos^2theta- sin^2theta)/(costheta - sintheta#
#=((costheta + sin theta)(costheta - sin theta))/(costheta - sin theta)#
#=((costheta + sin theta)cancel(costheta - sin theta))/cancel(costheta - sin theta)#
#=costheta + sintheta#
#LHS = RHS#
Hopefully this helps!