From Sine Law, →b =2R sin B............(i) →c = 2R sin C............(ii) Subtracting eq^{n} (ii) from (i) b - c = 2R(sin B - sin C)............(I) Adding eq^{n} (i) and (ii) b +c = 2R(sin B + sin C)............(II) Dividing eq^{n} (I) and (II)

This verifies the Tangent Law, the remaining result follow similar process.