Cos(x+y)* cos(x-y) =(cos(x+y- (x-y)) + cos(x+y+x-y) ) / 2 = (cos2y + cos2x) / 2.
-------
cos 82°30'cos37°30' =(cos(82°30'-37°30') +cos(82°30'-37°30') ) / 2 =
(cos45° +cos120°) / 2 =(√2/2 - 1/2) / 2 =(√2 - 1) / 4.
* * * * * * * преобразование произведения в сумму * * * * * * *
cosα*cosβ =(cos(α -β) +cos(α +β))/2