A = (-2, 1)
B = (5, 0)
C = (0, 5)
M_a = Midpoint(B, C)
M_b = Midpoint(A, C)
s_a = Line(A, M_a)
s_b = Line(B, M_b)
S = Intersect(s_a, s_b)
S1: (A + B + C) / 3