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# arithmetic seqeuences

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11. Find the partial sum S12 of the arithmetic series 26 + 24 +22+ 20+....

Sep 11, 2020

The formula is $$S_{n}=\frac{(T_1+T_n)n}{2}$$. Note that t_12=(26+2)-(2*12)=28-24=4. Note that t_1=26. So we have $$S_{12}=\frac{(4+26)12}{2}=\frac{30*12}{2}=30*6=\boxed{180}$$