[0 is less than or equal to pi which is less than 2pi]
5 Answers

cos (3x) = 1 –> 3x = arc cos 1
–> 3x = 0 + 2k pi with k Є N
–> x = (2/3)k pi
if k = 0 –> x = 0
if k = 1 –> x = (2/3) pi
if k = 2 –> x = (4/3) pi
A = { 0 ; (2/3) pi ; (4/3) pi }
Alejandra

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RE:
cos3x=1 ???
[0 is less than or equal to pi which is less than 2pi]

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Incorrect. When x=0, sin(2x) cos(3x)=0

Sketch the curve for cos x from 0 to 2pi. See where cos x is 0. Work backward from there.

Where does cos(x) = 1?
0 and 2π
So, of course still zero… (less than or equal to…)
and x = (2π)/3