i learn better by writing it down, so here we go:
nth term of arithmetic sequence: Tn = a + (n - 1)d
sum to n terms of arithmetic sequence: Sn = n/2[2a + (n - 1)d] or n/2(a + l)
nth term of geometric sequence: Tn = ar^n-1
sum to n terms of geo sequence: Sn = (a[1 - r^n])/(1 - r)
compound...