S9 = 9/2 (2a+8d) = 72
The sum of the next four terms is S13 - S9, so
13/2 (2a+12d) - 72 = 220
Solve for a
THE sum of the first 9 terms of an ap is 72 and the sum of the next 4 term is 220 find the first term
1 answer
1 answer