Question

2cosx+5=4

Answer 1

cos(x) = -1/2

Answer 2

2pi/3,
4pi/3

Answer 3

which one should I eliminate?

Answer 4

do I have to put the 2pin sign?

Answer 5

they are both the right answer

actually you have \(\dfrac{2\pi}{3}\pm 2\pi n\) and \(\dfrac{4\pi}{3}\pm 2\pi n\)

Answer 6

if cos(x)=a then cos(x) = cos(x+2pi) = cos(x+4pi)=...=a