Просто подставьте ваши числа тут ---
y' = [ (1+tgx)' (1-tgx) + (1-tgx)' (1+tgx) ] / (1-tgx)^2 =
= [ 1/cos^2 (x) (1-tgx)+1/cos^2 (1+tgx) ] / (1-tgx)^2 =
= [ 1/cos^2 (x) ( 1-tg x+1+tgx) / (1-tgx)^2 ] =
= [ 2/ cos^2 * cos^2 (x) ] / (cosx-sinx)^2 =
= -2/sin2x