It is not possible to answer this question without information on whether the terms are of an arithmetic or geometric (or other) progression, and what the starting term is.
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You.... have to apply this formula! n(n+1)/2 and n is the no. of terms
Find the Sum to n terms of the series 5 5+55+555+ +n Terms
n*(n+1)
There is no formula that will sum n even numbers without further qualifications: for example, n even numbers in a sequence.
200, 20, 2, 0.2 Here you have 4 terms. Add them together, and you find the sum of these four terms. If you need to find the sum of some other terms, i.e 8 terms, then you can use the formula Sn = [a1(r^n - 1/(r - 1) where n = 8, a1 = 200, and r = 1/10.