import math

def calc_term(a, i, xi):
    num = ((-1)**(i+1)*a*math.sin(xi))
    den = xi/2
    return round(num/den, 4)

def sum4terms(a, x0):
    s = x = x0
    i = 0
    x = calc_term(a, i, x)
    s, i = s+x, i+1
    x = calc_term(a, i, x)
    s, i = s+x, i+1
    x = calc_term(a, i, x)
    s, i = s+x, i+1
    return round(s, 2)

def loop(a, x0, n):
    s = x = x0
    i = 0
    print(i, x, s)
    i += 1
    while n > 0:
        x = calc_term(a, i, x)
        s += x
        print(i, x, s)
        n -= 1
        i += 1
    print(round(s, 2))
    print('ciao')