Question

Someone explains me, please. I don't get it

Answer 1

The part I circle

Answer 2

@wio

Answer 3

The thing is summing through 1 to n, but since there are no terms in the variable n, it's just summing up a^2 n times. So na^2.

Answer 4

in the variable i, I meant.*

Answer 5

It's just like saying that\[\sum_{i =1}^{n}c = cn\]

Answer 6

Oh, I think I know how stupid I am. hehehe. Thank you very much