sin 3 x = sin ( 3 x + 2 π ) = sin [ 3 ( x + 2 π/ 3 ) ] = sin 3 x
This means "after the arc rotating three time of ( x + ( 2 π /3 ) ) , sin 3x comes back to its initial value"
So, the period of sin 3x is 2 π /3 .
