Prove that:
tanA+tanBtanA−tanB=sin(A+B)sin(A−B)
LHS:tanA+tanBtanA−tanB
=sinAcosA+sinBcosBsinAcosA−sinBcosB
=sinAcosB+cosAsinBcosAcosBsinAcosB−cosAsinBcosAcosB
=sinAcosB+cosAsinBcosAcosB−cosAsinB
=sin(A+B)sin(A−B)
∴tanA+tanBtanA−tanB=sin(A+B)sin(A−B)
Hence proved.