就是两边同时被3减去
3-[1/(a^2+1)+1/(b^2+1)+1/(c^2+1)]
=[1-1/(a^2+1)]+[1-1/(b^2+1)]+[1-1/(c^2+1)]
=a^2/(a^2+1)+b^2/(b^2+1)+c^2/(c^2+1)