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Đáp án `+` Giải thích các bước giải:

`a)` `A = (sqrt{5} - 2)(sqrt{5} + 2)`

`<=> A = (sqrt{5})^2 - 2^2`

`<=> A = 5 - 4 = 1`

Vậy `A = 1`

`b)` `B = (sqrt{45} + sqrt{63})(sqrt{7} - sqrt{5})`

`<=> B = sqrt{9}.(sqrt{7} + sqrt{5})(sqrt{7} - sqrt{5})`

`<=> B = 3.[(sqrt{7})^2 - (sqrt{5})^2]`

`<=> B = 3(7 - 5)`

`<=> B = 3.2 = 6`

Vậy `B = 6`

`c)` `C = (sqrt{5} + sqrt{3})(5 - sqrt{15})`

`<=> C = (sqrt{5} + sqrt{3})(sqrt{5}.sqrt{5} - sqrt{5}.sqrt{3})`

`<=> C = sqrt{5}.(sqrt{5} + sqrt{3})(sqrt{5} - sqrt{3})`

`<=> C = sqrt{5}.[(sqrt{5})^2 - (sqrt{3})^2]`

`<=> C = sqrt{5}.(5 - 3)`

`<=> C = 2sqrt{5}`

Vậy `C = 2sqrt{5}`

`d)` `D = (sqrt{32} - sqrt{50} + sqrt{27})(sqrt{27} + sqrt{50} - sqrt{32})`

`D =[sqrt{27} + (sqrt{32} - sqrt{50})].[(sqrt{27} - (sqrt{32} - sqrt{50})]`

`D = (sqrt{27})^2 - (sqrt{32} - sqrt{50})^2`

`D = 27 - (4sqrt{2} - 5sqrt{2})^2`

`D = 27 - (- sqrt{2})^2`

`D = 27 - 2`

`D = 25` 

Vậy `D = 25`

`#Gao`

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+) Giải thích : Hằng đẳng thức : `(a^2 - b^2) = (a + b)(a - b)`