sin2 20 + sin2 70 / 2(cos 2 69 + cos 2 21) = sec 60/k then k is ? Share with your friends Share 2 Ashutosh Verma answered this Answer : We have : Sin 2 20° + Sin 2 70°2 Cos 2 69° + Cos 2 21° = Sec 60°k , So ⇒Sin 2 20° + Sin 2 90° - 20° 2 Cos 2 90° - 21° + Cos 2 21° = Sec 60°k⇒Sin 2 20° + Cos 2 20° 2 Sin 2 21° + Cos 2 21° = Sec 60°k ( we know Sin 90°-θ = Cos θ and Cos 90°-θ = Sin θ )⇒12 1 = 2k ( we know Sin 2θ+ Cos 2 θ = 1 and Sec 60°= 2 ) ⇒12 = 2k ⇒k = 4 ( Ans ) 6 View Full Answer