(2) Mathematically, it can also be expressed as "step-by-step" symbol, and the calculation method of step-by-step is similar to that of symbolic. The formula is n(n+ 1)/2-step addition: "~" is used to indicate step addition. Such as the step-by-step addition of x, it is expressed as x~.

Sequential addition formula for even and odd numbers:

Odd-order addition:1+3+5+7+… n = (n+1) (n+1)/4.

Even order addition: 2+4+6+8+…n=(n/2)(n/2)+n/2.

Hope to adopt, thank you! ! ! ! ! !