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If s in A=(12)/(13)a n dsinB=4/5,\ w h e...

If s in A=(12)/(13)a n dsinB=4/5,\ w h e r e\ pi/2 find cos a+b

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RHS`=(1)/(2)[tan27x-tanx]`
`=(1)/(2)[(tan27x-tan9x)+(tan9x-tan3x)+(tan3x-tanx)]`
`=(1)/(2)[((sin27x)/(cos27x))-(sin9x)/(cos9x))`
`+((sin 9x)/(cos9x)-(sin3x)/(cos3x))+((sin3x)/(cos3x)-(sinx)/(cos x))]`
`=(1)/(2)[(sin(27x-9x))/(cos 27x cos 9x)+(sin(9x-3x))/(cos 9xcos 3x)+(sin(3x-x))/(cos 3xcosx)]`
`=(1)/(2)[(sin18x)/(cos27x cos 9x)+(sin 6x)/(cos9x cos 3x)+(sin2x)/(cos 3xcos x)]`
`=(1)/(2)[(2sin 9xcos 9x)/(cos 27xcos9x)+(2sin 3x cos 3x)/(cos9x cos 3x)+(2sin x cos x)/(cos 3x cosx)]`
`=(sin9x)/(cos 27x)+(sin3x)/(cos9x)+(sinx)/(cos 3x)`
`=(sin x)/(cos 3x)+(sin3x)/(cos9x)+(sin9x)/(cos 27x)=LHS`.
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