The sum of the 8 terms is 160 while the sum of 20 term is 880;find the 43 terms,the sum of 12 terms,find their common difference,find the first term ,the sum of 60 terms
since you are using "common difference" in your text, I must assume
that you have an arithmetic sequence
sum of 8 terms is 160
---> (8/2)(2a + 7d) = 160
2a + 7d = 40
the sum of 20 term is 880
--->
(20/2)(2a + 19d) = 880
2a + 19d = 88
subtracting those 2 equations:
12d = 48
d = 4
subbing into 2a + 7d = 40
2a = 20
a = 10
use the sum formula to find sum(60)