Can someone explain this Riemann sum?

"Evaluate the integral by computing the limit of riemann sums"

Interval: 0-1 (bottom to top, I dont know how to do the integral symbol)

2x dx

first thing I do is figure out the change in x, which is 1/n (1-0)/n

next thing, I figure out x sub i, which is i/n (dxi)

then my book has me write out the formula, bear with me

(1/n)(summation symbol i=1 with the upperbound as n)(2(i/n))

then, seemingly through magic, it becomes (2/n^2)(n(n+1))/2

I say magic, because it just pretends like its immediately obvious whats going on. I know that im supposed to use that summation formula, no problem understanding that, but I dont understand how it got to 2/n^2

Also, how do I know which summation formula to use?