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## Answers

Therefore, the The factor " (First term + last term)/2" means the middle term of the A.P.

As the middle term of an AP having "n" terms with the first term as "a" and the common difference to be "d". = a+[({n+1}/2)-1]d..........(eqn. 1)

[the middle term of an odd no. series is given by (n+1)/2 ]

Similarly, the avg. of the first and last term is given by = [a+a+(n-1)d]/2

As, Clearly seen both the terms are same equal . Therefore, the sum can be found by this ; n×(middle term) = 13×40= 520.

Hence, 520 is the answer.