Question

A partial sum of an arithmetic sequence is given. find the sum. 5 + 9 + 13 + . . . + 441

Answer 1

Hello : 
A partial sum of an arithmetic sequence is : 
Sn = n/2(U1+Un)
U1 = 5   d  : the common difference is : 13-9 = 9-5 =4
calculate : n 
Un = U1 + (n-1)d
441 = 5 +(n-1)(4)
441 = 5 + 4n - 4  
4n = 440
n =110
so : 
 5 + 9 + 13 + . . . + 441 = 110/2(5+441) = 24530