Addition et soustraction

a + a = 2 x a = 2a
(-a) + (-a) = -a - a = -2a
a + (b - c) = a + b - c
a - (b + c) = a - b - c
a - (b - c) = a - b + c
a + (b - c + d) = a + b - c + d
a - (b - c + d) = a - b + c - d