Question

the sum of the first fifteen terms of an arithmetical progression is 105 and the sum of the next fifteen terms is 780. find the first three terms of the arithmetical prgoression.

Answer 1

in an arithmetic progression there are two parameters, those are

a = the starting value
d= the difference between two concecutive values so from that you can derive any value on the series by

\(a_n = a + (n-1)d\)

Since you know about these series im sure you also know how to take the sum of n conecutive points in a series using that twice youll be able to get two equations since there are ony two unknowns (a and d) using those two equations find the values for those two parameters.

Use above equation to find the first three values on the series