dy/dx=sec²(x+y)*(1+dy/dx)则[1-sec²(x+y)]dy/dx=sec²(x+y)
则dy/dx=sec²(x+y)/[1-sec²(x+y)]=1/cos²(x+y)÷[1-1/cos²(x+y)]=1/(cos²(x+y)-1)=-1/sin²(x+y)