Question 874159
It helps if you understand what they mean and have used them a few times.
Arithmetic 
if you know the last term use  S = ½(a + an)n
S = ½(2a + (n-1)d)n
an = a + (n - 1)d
  
Geometric 
Tn = t * r^(n - 1)
S=t*(1 - r^n)/(1 - r)