Question

Please solve the problem C.

S7 is the sum to the arithmetic series of 7 terms.

I've been trying to do it, but I get to the point when the number of terms is 25.6, which is obviously incorrect.

Answer 1

Sure.

The common difference is 1/3

d = 8/15 - 1/5 = 8/15 - 3/15 = 5/15 = 1/3

d = 13/15 - 8/15 = 5/15 = 1/3

So the seventh term is

S_n = S_1 + (n-1)d

S_7 = (1/5) + 6(1/3) = 11/5

The sum is the average of the first and last times the number of elements

(1/2)(1/5 + 11/5)(7) = 7(6/5) = 42/5 = 8.4

Answer: 42/5