To find the sum of the arithmetic sequence we have to use the formula sn=n/2(a1+an).
A1 is the value of the first term, n is the number of terms, and an is the value of the last term.
If the number of terms is known then we will use this formula. If the number of terms is not known, we need to find for n, by using the formula an=a1+(n-1)d.
First get the value of d, by subtracting any two consecutive terms, and then you may get the value of n.
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