Question

Summation and series notation.

Answer 1

your series will look like (plug in 1,2,3,4,5,6 for k)
\[1,~2,~3,~4,~5,~6,~7,~8,~9~... ...~99~.\]

to find the sum (being that it is an Arithmetic sequence, since d=+1)

\(\large \color{blue}{ \bf \frac{1}{2}(last~term~~+~~first~~term) (number~~of~~terms) }\)

Answer 2

1/2(first/last) is the average term

and then the obvious part - number of terms.