Đáp án+ Giải thích các bước giải:
`1/(\sqrt{10}+\sqrt{15}+\sqrt{14}+\sqrt{21})`
`=(\sqrt{10}+\sqrt{15}-\sqrt{14}-\sqrt{21})/((\sqrt{10}+\sqrt{15}+\sqrt{14}+\sqrt{21})(\sqrt{10}+\sqrt{15}-\sqrt{14}-\sqrt{21})`
`=(\sqrt{10}+\sqrt{15}-\sqrt{14}-\sqrt{21})/((\sqrt{10}+\sqrt{15})^2-(\sqrt{14}+\sqrt{21})^2)`
`=(\sqrt{10}+\sqrt{15}-\sqrt{14}-\sqrt{21})/(25+10\sqrt{6}-35-14\sqrt{6})`
`=(\sqrt{10}+\sqrt{15}-\sqrt{14}-\sqrt{21})/(-10-4\sqrt{6})`
`=(-\sqrt{10}-\sqrt{15}+\sqrt{14}+\sqrt{21})/(10+4\sqrt{6})`
