急, 我有d maths 啊大家可唔可以教吓我啊

2.
x² + 4x = 0
(x)²+2(x)(2)+(2)²-(2)²=0
(x+2)²=4
x+2=2 或x+2=-2
x=0 或x=-4

4c) 用配方法,求極值的x就是對稱軸
y = x² - 8x + 5
y=(x)²-2(x)(4)+(4)²-(4)²+5
y=(x-4)²-11
對稱軸為4

5a)
y = (x + 2)² + 4
x=-2時,極小值為4

10.
長方形面積
=x(24-2x)
=24x-2x²
=-2[(x)²-2(x)(6)+(6)² ]+2(6)²
=-2(x-6)² +72
x=6時,最大可能面積是72cm²

2)
x^2+4x = 0
(x^2 + 4x + 4) - 4 = 0
(x + 2)^ 2 = 4
x + 2 = 2 or x + 2 = -2
x = 0 or -4

4c)
y = x^2 - 8x + 5
= (x^2 - 8x + 16) - 16 + 5
= (x - 4)^2 - 11
對稱軸 : x = 4

5a)
y = (x+2)^2 + 4
極小值 : 4

10)
設長方形面積為Acm^2
A= x * (24-2x)
= 24x - 2x^2
= -2(x^2 - 12x)
= -2(x^2 - 12 + 36 - 36)
= -2(x^2 - 12 + 36) + 72
= -2(x-6)^2 + 72

最大可能面積是 72cm^2

(2) x^2 + 4x = [x^2 + 4x + (4/2)^2] - (4/2)^2
= (x + 2)^2 - 4

4c) y = x^2 - 8x + 5
= [x^2 - 8x + (8/2)^2] + 5 - (8/2)^2
= (x - 4)^2 - 11
對稱軸 x = 4

5a) y = (x + 2)^2 + 4
極小值 = 4

(10) (24 - 2x)x
= -2x^2+24
= -2[x^2 - 12 + (12/2)^2] + 2*(12/2)^2
= -2(x-6)^2 + 72
最大面積是 72 m^2

2008-10-27 22:14:36 補充：
(10) (24 - 2x)x
= -2x^2+24x
= -2[x^2 - 12x + (12/2)^2] + 2*(12/2)^2
= -2(x-6)^2 + 72
最大面積是 72 m^2