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Prove that cos^2 45^@-sin^2 15^@=sqrt3/4...

Prove that `cos^2 45^@-sin^2 15^@=sqrt3/4`

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`cos^2 45^@-sin^2 15^@`
`=cos(45^@+15^@)cos(45^@-15^@)`
`=cos60^@cos30^@`
`sqrt3/4`
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