import math


def getXY120(i, j):
    """
    Get xy coords for i,j indices in fig 2.4 DIF3D manual.

    Assumes triangleside length 1.0
    """
    height = math.sqrt(3) / 2.0
    y = j * height
    x = -0.5 * j + 0.5 * i
    return x, y