cos105' cos5'+sin105' sin5'/ cos18' cos62'-sin 62' cos72' =
=cos(105 -5) / cos18 cos62 - sin 62 cos(90 - 18) =
=cos100 /cos18' cos62 - sin 62 sin18 =
=cos100 /cos(62 + 18) =
=cos100 /cos80 =
=cos(90 +10) /cos(90-10) =
=-sin10 /sin10 =-1
cos105' cos5'+sin105' sin5'/ cos18' cos62'-sin 62' cos72' =
=cos(105 -5) / cos18 cos62 - sin 62 cos(90 - 18) =
=cos100 /cos18' cos62 - sin 62 sin18 =
=cos100 /cos(62 + 18) =
=cos100 /cos80 =
=cos(90 +10) /cos(90-10) =
=-sin10 /sin10 =-1