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2 cos^2 B - 1 = tan^2 A, then cosA cosB ...

` 2 cos^2 B - 1 = tan^2 A`, then `cosA cosB` =

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`2cos^2B - 1 = tan^2A`
`=>2cos^2B - 1 = sec^2A-1`
`=>2cos^2B = sec^2A`
`=>2cos^2B = 1/cos^2A`
`=>cos^2Acos^2B = 1/2`
`=>cosAcosB = +-1/sqrt2`
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