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`6.`

Xét `VT=1/(3+\sqrt{2})+1/(3-\sqrt{2})`

`=(3-\sqrt{2})/((3+\sqrt{2})(3-\sqrt{2}))+(3+\sqrt{2})/((3-\sqrt{2})(3+\sqrt{2}))`

`=(3-\sqrt{2}+3+sqrt{2})/((3+\sqrt{2})(3-\sqrt{2}))`

`=6/(3^2 -(\sqrt{2})^2)=6/(9-2)=6/7=VP` ( dpcm )