L.H.S = (1+cot A+tan A).(sin A−cos A)
=(1+cos Asin A+sin Acos A)(sin A−cos A)
=(sin A cos A+cos2A+sin2Asin A.cos A)(sin A−cos A)
=sin3A−cos3Asin A.cos A
[Using a3−b3=(a−b)(a2+ab+b2)]
=sin3Asin3A.cos3A−cos3Asin3A.cos3Asin A cos Asin3A.cos3A
[Dividing numerator and denominator by sin3.Acos3A]
=sec3A−cosec3Asec2A.cosec2A=R.H.S