>>895
1/cos(s) = (cos(s))/(cos(s)^2) = (cos(s))/(1-sin(s)^2)
= (1/2) {cos(s)/(1+sin(s))} + (1/2){ cos(s)/(1-sin(s))}

∫{1/cos(s) } ds = (1/2) log(1+sin(s)) -(1/2)log(1-sin(s))
= (1/2) log((1+sin(s))/(1-sin(s)))