Additive commutative law: The two addends are interchangeable, and the sum remains the same. A+B = B+A 40+56 = 56+40 Addition Law: Add the first two numbers or the last two numbers first, and the sum is unchanged. (a+b)+c = a+| b+c | 69+172 |+28 = 69+|172+28 | Introduction to Gaussian Algorithm Gauss was naughty when he was a child. When the teacher came back, he flew into a rage and ordered the whole class to start counting the scores of 1+2+3+4+5+6+ ...+100. Only (1+100)+(2+99)+(3+ 1+2+3+…+98+99+100 = (1+100)+(2+99)+(3+98) …+(50+565438) (the first item+the last item) × the number of items ÷ 21+2+3+...+98+99+100 = (1+100). That is, the last item-the first item-