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Prove that: 108sinpi/(18)-144sin^3pi/(18...

Prove that: `108sinpi/(18)-144sin^3pi/(18)=18`

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LHS : `108sin(pi/18)−144sin^3(pi/18)`
`=36xx3sin(pi/18)-36xx4sin^3(pi/18)`
`=36[3sin(pi/18)−4sin^3(pi/18)]`
`=36sin(3xx(pi/18))`
`=36sin(pi/6)`
`=36xx1/2=18`
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