令t=x+y

y=t-x

dy=d(t-x)

d(t-x)/dx=t^2

dt/dx-1=t^2

dt/dx=t^2+1

dt/(t^2+1)=dx

arctant=x+c

arctan(x+y)=x+c

所以正确答案应该是:

arctan(x+y)=x+c