y'=sec²(x+y)*(1+y')
y'[1-sec²(x+y)]=sec²(x+y)
y'=sec²(x+y)/[1-sec²(x+y)]
=-sec²(x+y)*cot²(x+y)
=-1/sin²(x+y)
即dy/dx=-1/sin²(x+y)
所以dy=-dx/sin²(x+y)