dy(1+tany)=(x^2+1)dxdy+siny/cosy* dy=(x^2+1)dxdy-d(cosy)/cosy=(x^2+1)dx积分：y-ln|cosy|=x^3/3+x+C代入y(0)=0,得：C=0所以特解为 y-ln|cosy|=x^3/3+x
