cos3x=1 ???

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

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