`8cos 10^@ . cos20^@ . cos 40^@`

`=(8cos 10^@ . cos20^@ . cos 40^@ . sin10^@)/(sin10^@)`

`=(4.(2sin10^@ .cos10^@) .cos20^@ .cos 40^@)/(sin10^@)`

`=(4. sin20^@ . cos20^@ . cos40^@)/(sin10^@)`

`=(2.(2sin20^@ .cos20^@) .cos40^@ )/(sin10^@ )`

`=(2sin40^@ .cos40^@)/(cos80^@)`

`=(sin80^@)/(cos80^@)`

`=tan80^@`. 

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Công thức:

`(sin alpha)/(cos alpha)=tan alpha `

`sin alpha = cos (90-alpha)`

`cos alpha =sin(90-alpha)`

`sin2alpha=2sinalphacosalpha`