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Prove that cos theta + cos (120^(@) + t...

Prove that ` cos theta + cos (120^(@) + theta) + cos ( 120^(@)-theta) =0. `
Hence deduce that `cos ^(3) theta + cos ^(3) ( 120^(@) + theta ) + cos^3( 120^(@)-theta) = 3/4 cos 3 theta.`

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