The sum of all five terms of an arithmetic sequence is

Question

55. Find the sum of the first three terms of the sequence if the sum of the last three is 48.

Answer 1

S5 = 5/2 (2a+4d)
S2 = 2/2 (2a+d)
The sum of the last 3 is S5-S2, so we have
S2 = 55-48 = 7 = 2a+d

2a+d = 7
5/2 (2a+4d) = 55

a=1, d=5

So, S3 = 3/2 (2+2*5) = 18