範例18A

 

 

x, y為實數, x2+y2=5, x+2y之最大值

[解法]

<Sol.>
∵(x+2y)2£(12+22)(x2+y2)
      (x+2y)2
£5´5=25
      |x+2y|
£5
     
-5£x+2y£