Đáp án:
`7)` `(\sqrt{28}-\sqrt{12}-\sqrt{7}).\sqrt{7}+2\sqrt{21}=7`
`8)` `(\sqrt{99}-\sqrt{18}-\sqrt{11}).\sqrt{11}+3\sqrt{22}=22`
Giải thích các bước giải:
`7)` `(\sqrt{28}-\sqrt{12}-\sqrt{7}).\sqrt{7}+2\sqrt{21}`
`=(\sqrt{2^2 . 7}-\sqrt{2^2 . 3}-\sqrt{7}).\sqrt{7}+2\sqrt{21}`
`=(2\sqrt{7}-2\sqrt{3}-\sqrt{7}).\sqrt{7}+2\sqrt{21}`
`=(\sqrt{7}-2\sqrt{3}).\sqrt{7}+2\sqrt{21}`
`=\sqrt{7}.\sqrt{7}-2\sqrt{3}.\sqrt{7}+2\sqrt{3}.\sqrt{7}`
`=7`
Vậy: `(\sqrt{28}-\sqrt{12}-\sqrt{7}).\sqrt{7}+2\sqrt{21}=7`
$\\$
`8)` `(\sqrt{99}-\sqrt{18}-\sqrt{11}).\sqrt{11}+3\sqrt{22}`
`=(\sqrt{3^2 . 11}-\sqrt{3^2 .2}-\sqrt{11}).\sqrt{11}+3\sqrt{22}`
`=(3\sqrt{11}-3\sqrt{2}-\sqrt{11}).\sqrt{11}+3\sqrt{22}`
`=(2\sqrt{11}-3\sqrt{2}).\sqrt{11}+3\sqrt{22}`
`=2\sqrt{11}.\sqrt{11}-3\sqrt{2}.\sqrt{11}+3\sqrt{2}.\sqrt{11}`
`=2.11+0=22`
Vậy `(\sqrt{99}-\sqrt{18}-\sqrt{11}).\sqrt{11}+3\sqrt{22}=22`
