giúp mình với sắp đi học rồi

`a)`

`A=\sqrt{4+2\sqrt{3}}+\sqrt{4-2\sqrt{3}}`

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

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

`=|\sqrt{3}+1|+|\sqrt{3}-1|`

`=\sqrt{3}+1+\sqrt{3}-1`

`=(\sqrt{3}+\sqrt{3})+(1-1)`

`=2\sqrt{3}`

`b)` `B=x-4+\sqrt{16-8x+x^2}`           Với `x>4`

`=x-4+\sqrt{4^2-2.4.x+x^2}` 

`=x-4+\sqrt{(4-x)^2}` 

`=x-4+|4-x|`

`=x-4+x-4`   ( Vì `x>4` )

`=(x+x)+(-4-4)`

`=2x-8`

`c)` `C=((\sqrt{14}-\sqrt{7})/(1-\sqrt{2})+(\sqrt{15}-\sqrt{5})/(1-\sqrt{3})):1/(\sqrt{7}-\sqrt{5})`

`=[(\sqrt{7}.(\sqrt{2}-1))/(1-\sqrt{2})+(\sqrt{5}.(\sqrt{3}-1))/(1-\sqrt{3})].(\sqrt{7}-\sqrt{5})`

`=[(-\sqrt{7}.(1-\sqrt{2}))/(1-\sqrt{2})-(\sqrt{5}.(1-\sqrt{3}))/(1-\sqrt{3})].(\sqrt{7}-\sqrt{5})`

`=-[(\sqrt{7}+\sqrt{5}).(\sqrt{7}-\sqrt{5})]`

`=-[(\sqrt{7})^2-(\sqrt{5})^2]`

`=-(7-5)`

`=-2`