GMAT Math

I've just seen these formulas on the "Quant revision" notes downloaded from the GMAT wiki.

Can someone please explain what they mean and when they should be used? I've gone through all questions in the OG guide and do not seem to have needed to use any of these formulas.

What do the variables "a", "r" and "n" in the equations stand for? Thanks!

AP: An = a + (n-1)d
Sn= n/2[2a+(n-1)d]

GP: An= a[r^(n-1)]
Sn= a[(r^n)-1]/r-1

Sum of squares of 1st n natural numbers: n(n+1)(2n+2)/6

Sum of cubes of 1st n natural numbers: [n(n+1)/2]^2

Check this link for some explanation....
https://rbmix.com/math/progressions/seq.html

Check this link for some pblms...
https://questionbank.4gmat.com/mba_prep_ ... ons/ap_gp/

thanks for the links jangojess!
-Ashish

Yes JangoJess these links are immensely helpful, a thousand thanks.