These can be easily understood by converting into the degrees
`theta = 2pi/3` = `2/3 ` of `180^@` `= 120 ^@`
`theta '` = the reference angle
The reference angle is defined as
"The smallest angle that the terminal side of a given angle makes with the x-axis"
so the reference angle is `theta '` = `60^@` which is shown in the attachment below
sorry the pic didnt upload so, i had to upload it again