Question

sin^2(35)-cos^2(145)/cos100*cos350

Answer 1

(Sin²35 - Cos²145) / Cos100*Cos350 = [Sin²35 - Cos²(180 - 35)] / Cos(90 +  +10)*Cos(360 - 10) = (Sin²35 - Cos²35) / (- Sin10)*Cos10 = 
= - Cos 20 / (- Sin10Cos10) = 2Cos20 / Sin20 = 2 ctg20