Đáp án :
`A = -2\sqrt3`
`B = 2`
Giải thích các bước giải :
`b)`
`A = \frac{ 4} { \sqrt5 + \sqrt3} - \sqrt{20}`
`A = \frac{4}{ \sqrt5 + \sqrt3 } - \sqrt{ 4 . 5 } `
`A = \frac{ 4 ( \sqrt5 - \sqrt3 ) }{ ( \sqrt5 + \sqrt3 ) ( \sqrt5 - \sqrt3 )} - 2\sqrt5`
`A = \frac{ 4 ( \sqrt5 - \sqrt3 ) }{ 5 - 3 } - 2\sqrt5`
`A = \frac{ 4 ( \sqrt5 - \sqrt3 ) }{ 2 } - 2\sqrt5`
`A = 2 ( \sqrt5 - \sqrt3 ) - 2\sqrt5`
`A = 2\sqrt5 - 2\sqrt3 - 2\sqrt5`
`A = -2\sqrt3`
Vậy `A = -2\sqrt3`.
`B = ( 1 + \sqrt3 ) . \sqrt{ 4 - 2\sqrt3}`
`B = ( 1 + \sqrt3 ) . \sqrt{ 3 - 2\sqrt3 + 1 }`
`B = ( 1 + \sqrt3 ) . \sqrt{ ( \sqrt(3)^2) - 2 . \sqrt3 . 1 + 1^2`
`B = ( 1 + \sqrt3 ) . \sqrt{( \sqrt3 - 1 )^2}`
`B = ( 1 + \sqrt3 ) . | \sqrt3 - 1 | `
`B = ( 1 + \sqrt3 ) . ( -1 + \sqrt3) `
`B = -1 + \sqrt3 - \sqrt3 + 3`
`B = 2`
Vậy `B = 2`.
