`a)`
`( \sqrt{6} + \sqrt{10})/(\sqrt{21} + \sqrt{35})`
` = ( \sqrt{2} (\sqrt{3} + \sqrt{5}))/(\sqrt{7} (\sqrt{3} + \sqrt{5}))`
` = (\sqrt{2})/(\sqrt{7})`
`= (\sqrt{14})/7`
`b)`
`(\sqrt{6} + \sqrt{4})/(2 \sqrt{3} + \sqrt{28})`
` =(\sqrt{2} (\sqrt{3} + \sqrt{2}))/(2 \sqrt{3} + 2 \sqrt{7})`
` = (\sqrt{2} (\sqrt{3} + \sqrt{2}))/( 2 (\sqrt{3} + \sqrt{7}))`
`= (\sqrt{3} + \sqrt{2})/( \sqrt{2} (\sqrt{3} + \sqrt{7}))`
`= (( \sqrt{3} + \sqrt{2})(\sqrt{3} - \sqrt{7}))/( \sqrt{2} (\sqrt{3} + \sqrt{7})(\sqrt{3} - \sqrt{7}))`
` = (\sqrt{6} - \sqrt{14} - \sqrt{21} + 3)/(-4 \sqrt{2})`
` = (-\sqrt{2} (\sqrt{6} - \sqrt{14} - \sqrt{21} + 3))/(4 \sqrt{2} . \sqrt{2})`
` = ( -3 \sqrt{2} - 2 \sqrt{3} + 2 \sqrt{7} + \sqrt{42})/8`
