设x+y =b 代入：(x/4)平方+（y/5）平方=1得：
1/16*x^2+1/25*b^2-2/25*bx+1/25*x^2 =1
(1/16+1/25)x^2 - 2/25 bx +1/25 b^2-1 =0
△ = 4b^2/25^2 -4(1/16+1/25)(1/25 b^2 -1) ≥ 0
-√41 ≤ b ≤ √41
即：x+y 的最大值 = √41 ；x+y的最小值 = -√41