"the 13th term is six times the second" ---> a + 12d = 6(a+d)
a + 12d = 6a + 6d
6d = 5a
d = 5a/6
"the sum of the first 15 terms is 615"
---> (15/2)(2a + 14d) = 615
2a + 14d = 82
a + 7d = 41
use substitution:
a + 7(5a/6)= 41
6a + 35a = 246
41a = 246
a = 6 , then d = 5/6 (6) = 5
so you have 6 , 11, 16, ...
No idea what the "Geometric Progression" comment at the end of
your post is supposed to mean.
In an arithmetic sequence the sum of the first 15 terms is 615 and the 13th term is six times the second. Find the first three terms.
Geometric Progression
1 answer