>>580
y = log √{(1-cos(x))/(1+cos(x))}
exp(y) = √{(1-cos(x))/(1+cos(x))}
exp(2y) = {(1-cos(x))/(1+cos(x))} = -1 + {2/(1+cos(x))}
の両辺をxで微分して

2y' exp(2y) = 2 sin(x)/ (1+cos(x))^2

あとは
exp(2y) = {(1-cos(x))/(1+cos(x))} を代入して整理