Answer:
a +7d = 11
Step-by-step explanation:
The sum of an arithmetic series is given by the formula ...
Sn = (n/2)(2·a1 +d(n -1))
where n is the number of terms, a1 is the first term, and d is the common difference.
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Here, you are given n=15, S = 165, a1=a, and asked to find a relation for 'a' and 'd'. Substituting the known values, we have ...
165 = (15/2)(2a +d(14))
11 = a +7d . . . . . . . . simplify and divide by 15
We have found the relation to be ...
a +7d = 11 . . . . . as required
