Posted by Ashley on .
The sum of the first 4 terms of an arithmetic series is 8 and the sum of the first 5 terms is 85. Determine the first term and the common difference

arithmetic 
Reiny,
So Sum(5)  Sum(4) = term(5)
85(8) = a + 4d
a+4d = 93 < (#1)
Sum(4) = 8 , since there are only 4 terms I will not use the sum formula
a + a+d + a+2d + a+3d = 8
4a + 6d = 8
2a + 3d = 4 , (#2)
2(#1)  (#2) > 5d = 190
d = 38
in #1, a = 59
check:
59  21 + 17+ 55 = 8
sum(5) = sum(4) + term(5) = 8 + (59+4(38)) = 85