Diện tích xung quanh ban đầu là:

`S=4 xx a^2`

Diện tích xung quanh sau khi thay đổi là:

`S' = 4 xx (2 xx a)^2 = 4 xx 4 xx a^2= 16xx a^2`

Diện tích xung quanh gấp lên số lần là:

`S' : S = 16 xx a^2 : 4 xx a^2= 4` (lần)

Đáp số: `4` lần.

c) `1/3 \sqrt51 = \sqrt( (1/3)^2 .51) =\sqrt(17/3)`

`1/5 \sqrt150 =\sqrt( (1/5)^2 .150)=\sqrt6`

`=> \sqrt(17/3) < \sqrt6`

`=> 1/3 \sqrt51 < 1/5 \sqrt150`

d) `1/2 \sqrt6 = \sqrt(3/2)`

`6\sqrt(1/2) =\sqrt(18)`

`=> \sqrt(3/2) < \sqrt18`

`=> 1/2 \sqrt6 < 6\sqrt(1/2)`.

a) `3\sqrt3=\sqrt(3^2 .3)=\sqrt27`

\sqrt12=\sqrt12`

`=> \sqrt27 > \sqrt12`

`=> 3\sqrt3 > \sqrt12`

b) `7=\sqrt(7^2)=\sqrt49`

`3\sqrt5=\sqrt(3^2 .5)=\sqrt45`

`=> \sqrt49>\sqrt45`

`=>7>3\sqrt5`

`[16(x+y)^5-12(x+y)^3]: 4(x+y)^2`

`=16(x+y)^5 : 4(x+y)^2 - 12(x+y)^3 :4(x+y)^2`

`=4(x+y)^3-3(x+y)^2`

`=(x+y)^2 )[4(x+y)-3]`

`=(x+y)^2 (4x+4y-3)`

37. B chứ nhỉ?

38. C 

39. A

33. B
34. C
35. A
36. D
37. B
38. A
39. B
40. D

a) 

\(A=\left(\dfrac{\sqrt{x}-2}{\sqrt{x}+2}-\dfrac{8\sqrt{x}}{4-x}\right):\dfrac{\sqrt{x}+2}{1-2\sqrt{x}}\\ =\left(\dfrac{\left(\sqrt{x}-2\right)^2}{\left(\sqrt{x}+2\right)\left(\sqrt{x}-2\right)}+\dfrac{8\sqrt{x}}{\left(\sqrt{x}+2\right)\left(\sqrt{x}-2\right)}\right).\dfrac{1-2\sqrt{x}}{\sqrt{x}+2}\\ =\dfrac{x-4\sqrt{x}+4+8\sqrt{x}}{\left(\sqrt{x}+2\right)\left(\sqrt{x}-2\right)}.\dfrac{1-2\sqrt{x}}{\sqrt{x}+2}\\ =\dfrac{x+4\sqrt{x}+4}{x-4}.\dfrac{1-2\sqrt{x}}{\sqrt{x}+2}\\ =\dfrac{\left(\sqrt{x}+2\right)^2.\left(1-2\sqrt{x}\right)}{\left(\sqrt{x}+2\right)^2.\left(\sqrt{x}-2\right)}\\ =\dfrac{1-2\sqrt{x}}{\sqrt{x}-2}\)

b) `A=-1/3`

`<=> (1-2\sqrtx)/(\sqrtx-2)=-1/3`

`<=> 3(1-2\sqrtx)=-1(\sqrtx-2)`

`<=>3-6\sqrtx=-\sqrtx+2`

`<=>-5\sqrtx=-1`

`<=>\sqrtx=1/5`

`<=>x=1/25`

`(2^39 +2^35): 2^33`

`=2^39 : 2^33 + 2^35 :2^33`

`=2^(39-33)+2^(35-33)`

`=2^6+2^2`

`=68`