How to find reference angle of 28pi/18

I'm a big fan of converting radians to degrees so the angles are easier to plot in a coordinate plane. Converting the radian measure to degrees is done with dimensional analysis, using the fact that there are 180 degrees in pi. . Let's reduce that first. Now that's out of the way.... So 280 degrees. That angle will fall into the fourth quadrant, only 10 degrees "after" the negative y axis, which is 270. We go 10 degrees into the 4th quadrant. Reference angles are ALWAYS measured from the tip of the terminal ray of the angle to the nearest x axis. Since all 4 quadrants measure 90 degrees, 90 - 10 = 80. The reference angle, then, is 80 degrees. Converting that back to radians we get that the reference angle is . There you go!

yes because there is exactly one y value for every x value

for function there can't be more than one value for every x value