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Yahoo!奇摩知識+ - 分類問答 - 科學常識 - 已解決
Yahoo!奇摩知識+ - 分類問答 - 科學常識 - 已解決 
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算幾不等式&配方法
Aug 2nd 2014, 01:49

1.

(x^2+y^2)/2 xy

xy 2/2 = 1

 

2.

a > 0

(a+1/a)/2 (a)(1/a) = 1

a + 1/a 2

 

a < 0

a + 1/a = -((-a) +(-1/a))

(-a) +(-1/a) 2

-((-a) +(-1/a)) -2

a + 1/a -2

 

3.

μ= (1+2+3+4+5)/5 = 3

f(x)之最小值 = f(3) = 4+1+0+1+4 = 10

 

4.

x-1=(y+1/2)=(z-2/3) ??

假設為 x – 1 = (y+1)/2 = (z-2)/3

x = t + 1, y = 2t – 1, z = 3t + 2

 

x^2+y^2+z^2

=(t-1)^2 + (2t-1)^2 + (3t+2)^2

= 15t^2 + 6t + 6

=15(t^2 + 2t/5) + 6

=15(t+1/5)^2 + 6 – 15(1/25)

=15(t+1/5)^2 + 27/5

 

x^2+y^2+z^2之最小值為 27/5

 

 

5.

x^4+2x^2+5 = (x^2)^2 + 2(x^2) + 5 = (x^2 + 1)^2 +5 – 1

x^2 = 0, 最小值 = 1 + 5 -1 = 5

 

6.

x^2+2y^2=1, y^2 = (1-x^2)/2

1 – x^2 0

-1 x 1

 

2x+3y^2

= 2x + 3(1-x^2)/2

= -3x^2/2 + 2x + 3/2

=(-3/2)(x^2 – 4/3x) + 3/2

=(-3/2)(x – 2/3)^2 + 3/2 – (-3/2)(4/9)

=(-3/2)(x – 2/3)^2 + 13/6, -1 x 1

 

x = 2/3, 2x+3y^2之最大值為 13/6

x= -1, 2x+3y^2之最小值為 (-2/3)(-1-2/3)^2 + 13/6 = -2

 

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