import math

def tri(n):
    """nth triangular number"""
    return n * (n + 1) /2.0

def trilist(n):
    """n terms of the triangular number sequence"""
    thelist = []
    for i in range(1, n+1):
        thelist.append(tri(i))
    return thelist

def trilist2(n):
    """n terms of the triangular number sequence (alternate)"""    
    return [tri(i) for i in range(1, n+1) ]

def distance2d(p0, p1):
    """distance formula in xy plane"""        
    return math.sqrt((p0[0]-p1[0])**2 + (p0[1]-p1[1])**2)


def distance3d(p0, p1):
    """distance formula in xyz """        
    return math.sqrt((p0[0]-p1[0])**2 +
                     (p0[1]-p1[1])**2 +
                     (p0[2]-p1[2])**2)