Calculate the terms of the sequence

[Julia_26]

Let the sequence (xn) be defined using the summation notation:
[tex]x_n=(\sum_{k=1}^{n})=1+2+...+(n-1)+n[/tex]
Calculate the first four terms of the sequence
Thanks:)

[Math Tutor]

The sum of the first four terms is:

(1) + (1+2) + (1+2+3) + (1+2+3+4) = 1 + 3 + 6 + 10 = 20