$49-54$ . A partial sum of an arithmetic sequence is given. Find the sum.
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in the partial. Some given this sigma notation so partial. Some formula is an over to number of terms over two times a one plus a ends a first term plus the last term and first term. So the number of terms is 21 because it's 20 here and starts with zero. So 21 divided by two times the first term first term is, uh, is equal to zero because when a musical 20 can get eyes Actually, what sorry is one one plus in the last term? A 20. So one minus 40 which is negative. 39. So nice thing you're 39. You play this into a calculator, we get negative 399 as the partial Some