import math

def perpendicular(l, c, r):
    # l stands for left, c for centre, r for right angles
    # the inputs are floats representing angles
    # returns the angle which is perpendicular to the screen
    float theta1
    dT1 = c - l
    dT2 = r - c

    # convert to radians
    dT1 = dT1 * math.pi / 180
    dT2 = dT2 * math.pi / 180

    # This is really only a theoretic case
    #if |dT2| < 0.001:
        #theta1 = math.pi/2

    # apply the formula to obtain theta1
    up = math.cos(dT1) * math.sin(dT1) * math.sin(dT2) - 1
    down = (math.sin(dT1) * math.sin(dT1)) * math.sin(dT2)

    theta1 = math.arctan(up / down)

    theta1 = theta1 * 180 / math.pi

    return (l - theta1)