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A tower subtends angles alpha,2alpha,3al...

A tower subtends angles `alpha,2alpha,3alpha` respectively, at point `A , B ,a n dC` all lying on a horizontal line through the foot of the tower. Prove that `(A B)/(B C)=1+2cos2alphadot`

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`/_APB=/_PBC-/_BAP`
`2alpha-alpha`
`/_APB=alpha`
`/_BPC=alpha`
`BP=AB`
Using Sin formula for triangle BCP
`(BC)/(sinalpha)=(AB)/(sin(180-3alpha))`
`(AB)/(BC)=(sin3alpha)/sinalpha`
...
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