Question 304115
A reference angle for a given angle in standard position is the positive acute angle formed by the x-axis and the terminal side of the given angle. Reference angles, by definition, always have a measure between 0 and pi/2  

The reference angle for 150 (quadrant II) is 180-150 = 30. The side opposite the 30 degree angle in quadrant II is positive so sin(150) = +sin(30)