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If cos theta = (1)/(2)(a + (1)/(a)),show...

If cos `theta = (1)/(2)(a + (1)/(a)),`show that cos 30 = `(1)/(2) (a^(3) + (1)/(a^(3)))`.

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The correct Answer is:
`(1)/(2)(a^(3) + (1)/(a^(3)))`
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