`a) 1/ (2 - \sqrt{3}) - 1/ (2 + \sqrt{3})`
` = (2 + \sqrt{3})/((2 -\sqrt{3}). (2 + \sqrt{3})) - (2 - \sqrt{3})/((2 -\sqrt{3}). (2 + \sqrt{3}))`
` = ((2 + \sqrt{3}) - (2 - \sqrt{3}))/((2 -\sqrt{3}). (2 + \sqrt{3}))`
` = (2 + \sqrt{3} - 2 + \sqrt{3})/(2^2 - (\sqrt{3})^2)`
` = (\sqrt{3} + \sqrt{3})/ (4-3)`
` = (2.\sqrt{3})/1`
` = 2\sqrt{3}`
`b)`
`2 \sqrt{40} - 5\sqrt{2} . 2 \sqrt{5} + 15 \sqrt{2/5} - (\sqrt{50})/(\sqrt{5})`
` = 2 . 2 \sqrt{10} - 5.2 .\sqrt{5} . \sqrt{2} + 15 . (\sqrt{2})/(\sqrt{5}) - \sqrt{50/5}`
` = 4 . \sqrt{10} - 10 .\sqrt{10} + (15 . \sqrt{10})/5 - \sqrt{10}`
` = 4 . \sqrt{10} - 10 .\sqrt{10} + \sqrt{10} . 3 - \sqrt{10}`
` = \sqrt{10} . (4-10 + 3-1)`
` = \sqrt{10} . (-4)`
` = -4\sqrt{10}`
