You want
40/2 (2a+39d)
where
12/2 (2a+11d) = 186
a+19d = 83
Just solve those last two for a and d, then plug them into the first formula.
The sum of first 12 terms of an arithmetic series is 186, and the 20th term is 83. What is the sum of first 40 terms?
1 answer