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cos(36^@-A)cos(36^@+A)+cos(54^@+A)cos(54...

`cos(36^@-A)cos(36^@+A)+cos(54^@+A)cos(54^@-A)`=`cos2A`

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`cos(36^@-A)cos(36^@-A)+cos(54^@+A)cos(54^@-A)`
`=cos(36^@-A)cos(36^@-A)+sin[90^@-(54^@+A)]sin[90^@-(54^@-A)]`
`=cos(36^@-A)cos(36^@-A)+sin(36^@-A)sin(36^@+A)`
`=cos(36^@+A-36^@-A)`
Using formula of `cos(A-B)`
`=cos 2A`
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