Ta có:
`\qquad {8cos20°}/{\sqrt{3}}.\sqrt{3}cos10°`
`=8cos20°.cos10°`
`=8. 1/2. [cos(20°+10°)+cos(20°-10°)]`
`=4.(cos30°+cos10°)`
`=4. (\sqrt{3}/2+cos10°)`
`=4cos10°+2\sqrt{3}`
`=>{8cos20°}/{\sqrt{3}}.\sqrt{3}cos10°=4cos10+2\sqrt{3}`
`=>{8cos20°}/{\sqrt{3}}={4cos10°+2\sqrt{3}}/{\sqrt{3}.cos10°}` (đpcm)