1/sinx的不定积分是ln|tan(x/2)|+C,不是tan(x/2)
省略积分号
dx/sinx=dx/(2sin(x/2)cos(x/2))
=(sec(x/2))^2d(x/2)/tan(x/2)
=d(tan(x/2))/tan(x/2)
=ln|tan(x/2)|+C