Cos5x = 1

1/cos5x=1
2/cos(x+pi/4)=1
3/cosx/2=-1
4/cos(pi/3-2x)=-1
5/cos(3x-pi/4)=0
6/cos(3x/5-pi/6)=0
7/cosx=1/2
8/cosx= căn 2/2
9/cosx=-1/2,x thuộc [-pi/2;3pi]
10/cosx= - căn 2/2
11/cosx= căn 3/2 
12/cosx=- căn 3/2
13/ 2cos(x-pi/6)=1
14/2cos(3x/2-pi/4)+1=0
17/2cos(x+60 độ) +căn 2=0
18/2cos(x-45 độ)-1=0
19/2cos(5-x)+1=0
20/2cosx+3=0