(3) 原式 = lim(x^2+y^2)/e^(x+y) = limr^2/e^[r(cost+sint)] = [1/e^(cost+sint)] limr^2/e^r = [1/e^(cost+sint)] lim2r/e^r = [1/e^(cost+sint)] lim2/e^r = 0 因 -√2
