The sum of the first 12terms of an arithmetic progression is 135 while the sum of the next 8 terms is 210. Find the common difference

4 answers

We are told that
S12 = 135
S20-S12 = 210
S20 - 135 = 210
S20 = 345

Now, we know that
S20 = 20/2 (2a+19d)
S12 = 12/2 (2a+11d)

So, we have

10(2a+19d) = 345
6(2a+11d) = 135

Solve that for d.

old how did you get 345

please how did you get 345

OK I see now . sorry I asked