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

A tower subtends angles `alpha,2alpha,3alpha` respectively, at point `A , B ` and `C` all lying on a horizontal line through the foot of the tower. Then `(A B)/(B C)`=

`(sin 3alpha )/(sin2alpha)`

`1+2 cos 2 alpha`

`2 cos 3 alpha`

`(sin 2 alpha )/(sin alpha ) `

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