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By considering that

tan⁡θ=sin⁡θcos⁡θ
  1. show that

    tan⁡(A+B)=tan⁡A+tan⁡B1−tan⁡Atan⁡B
  2. Write down the formula for

    tan⁡(A−B)

tan⁡(A+B)=sin⁡(A+B)cos⁡(A+B)=sin⁡Acos⁡B+cos⁡Asin⁡Bcos⁡Acos⁡B+sin⁡Asin⁡B

Now divide both the numerator and denominator of

sin⁡Acos⁡B+cos⁡Asin⁡Bcos⁡Acos⁡B+sin⁡Asin⁡B

by

cos⁡Acos⁡B