∑(a+b)=∑a+∑b∑(a + b) = ∑a + ∑b∑(a+b)=∑a+∑b
∑(a−b)=∑a−∑b∑(a - b) = ∑a - ∑b∑(a−b)=∑a−∑b
∑(ab)=b∑a=a∑b∑(ab) = b∑a = a∑b∑(ab)=b∑a=a∑b
