y=tan(x+y)y'=tan'(x+y)=[sec(x+y)]^2*(1+y')y'=[sec(x+y)]^2/{1-[sec(x+y)]^2}分子=1/[cos（x+y）]^2分母=1-1/[cos(x+y)]^2=-[tan(x+y)]^2=-[sin(x+y)]^2/[cos(x+y)]^2y'=-1/[sin(x+y)]^2=-[csc(x+y)]^2