giúp mình 18 22 23 thôi mọi người 50đ

Đáp án+Giải thích các bước giải:

18,

`frac{x^2-4}{\sqrt{x^2-4x+4}}(x<2)`

`=\frac{(x-2)(x+2)}{\sqrt{x^2-2.x.2+2^2}}`

`=\frac{(x-2)(x+2)}{\sqrt{(x-2)^2}}`

`=\frac{(x-2)(x+2)}{|x-2|}`

`=\frac{(x-2)(x+2)}{-(x-2)}`

`=-(x+2)`

`=-x-2`

22,

`\frac{\sqrt{x^4+10.x^2+25}}{x^2+5}`

`=\frac{\sqrt{x^4+2.x^2 .5+5^2}}{x^2+5}`

`=\frac{\sqrt{(x^2+5)^2}}{x^2+5}`

`=\frac{|x^2+5|}{x^2+5}`

`=\frac{x^2+5}{x^2+5}`

`=1`

23,

`\frac{\sqrt{x^4+6.x^2+9}}{x^2+3}-\sqrt{x^4+2x^2+1}`

`=\frac{\sqrt{x^4+2.x^2 .3+3^2}}{x^2+3}-\sqrt{x^4+2.x^2 .1+1^2}`

`=\frac{\sqrt{(x^2+3)^2}}{x^2+3}-\sqrt{(x^2+1)^2}`

`=\frac{|x^2+3|}{x^2+3}-|x^2+1|`

`=\frac{x^2+3}{x^2+3}-(x^2+1)`

`=1-x^2-1`

`=-x^2`