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By considering that

tanθ=sinθcosθ
  1. show that

    tan(A+B)=tanA+tanB1tanAtanB
  2. Write down the formula for

    tan(AB)

tan(A+B)=sin(A+B)cos(A+B)=sinAcosB+cosAsinBcosAcosB+sinAsinB

Now divide both the numerator and denominator of

sinAcosB+cosAsinBcosAcosB+sinAsinB

by

cosAcosB