2. [Nguồn : VMF]
4.
P = 3x + 2y + 6/x + 8/y
<=>P = (3x/2 + 6/x) + (3x/2 + 3y/2) + (y/2 + 8/y)
Ta có :
3x/2 + 6/x \geq 2\sqrt{ 3x/2.6/x} = 6
Dấu = xảy ra khi 3x/2 = 6/x <=> x = 2
3x/2 + 3y/2 = 3/2.(x+y) \geq 3/2.6 = 9
Dấu = xảy ra khi x + y = 6
y/2 + 8/y \geq 2.\sqrt{y/2.8/y} = 4
Dấu =...