Из дкр по . , .промежуток забыла, к сожалению. 3cos10x-8cos5x+5=0

3.cos 10x -8.cos 5x +5=0
3.cos 2.5x - 8.cos 5x +5=0
3.(cosˇ2 5x - sinˇ2 5x) -8.cos 5x +5 = 0
3.(cosˇ2 5x - (1-cosˇ2 5x))-8.cos 5x +5=0
3.(2. cosˇ2 5x -1) -8.cos 5x + 5 = 0
6.cosˇ2 5x -3 - 8.cos 5x + 5 = 0
6.cosˇ2 5x -8.cos 5x +2=0
3.cosˇ2 5x - 4.cos 5x + 1 = 0
cos 5x = z
3.zˇ2 -4z + 1 = 0
z1=1, z2=1/3
a)cos 5x = 1, 5x = 2k.pí, x = 2k.pí/5
b)cos 5x = 1/3 =priblizitelno 0,33333., 5x = 73graducov+2k.pí i 5x =287graducov+2k.pí, x = 73/5graducov + 2k.pí/5 i x = 287/5 graducov + 2k.pí/5