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Prove that: tan20^0tan40^0tan80^0=tan60^...

Prove that: `tan20^0tan40^0tan80^0=tan60^0`

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Given:---
`=>tan20^@tan40^@tan80^@`
`=>(sin20sin40sin80)/(cos20cos40cos80)`
Multiplying and dividing by `2` both sides
`=>(2sin20sin40sin80)/(2cos20cos40cos80)`
using formula,
`2sinAsinB=cos(A+B)cos(A-B)`
`=>((cos(20-40)-cos(20+40))sin80)/((cos(20+40)+cos(20-40))cos80)`
...
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