Đáp án:
`a, 1/(7 + 4\sqrt{3}) + 1/(7 - 4\sqrt{3})`
`= (7 - 4\sqrt{ 3} + 7+ 4\sqrt{3})/((7 + 4\sqrt{3})( 7 - 4\sqrt{3}))`
`= 14/(7^2 - (4\sqrt{3})^2)`
`= 14/( 49 - 48)`
`= 14`
`b, 3/(\sqrt{2} - 1) - 3/(\sqrt{2} + 1) `
`= (3\sqrt{2} + 3 - 3\sqrt{2} + 3)/((sqrt{2})^2 - 1)`
`= 6/1`
`=6`
`c, 1/(2+\sqrt{3} ) - 1/(2-\sqrt{3} )`
`= (2-\sqrt{3} - 2-\sqrt{3} )/(2^2 - (\sqrt{3})^2`
`= (-2\sqrt{3})/ 1`
`= -2\sqrt{3}`
