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![S= -1 -3 -5 -7 ...\\ \\ S=-(1+3+5+7+........)\\ \\ \text{This is an arithmetic series with first term=a=1 and }\\ \text{common diffrence=d=3-1=2}\\ \\ \text{Using the arithmetic series sum formula we can say}\\ \\ S_n=\frac{n}{2}[2a+(n-1)d]\\ \\ S_{15}=\frac{15}{2}[2*1+(15-1)*2]\\ \\ S_{15}=\frac{15}{2}[2+28]\\ \\ S_{15}=\frac{15}{2}*30\\ \\ S_{15}=225\\ \\ \text{Hence the sum of the given sequence is }= -225\\](/tpl/images/0086/8437/fcc61.png)
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Find s15 for the series -1 + -3 + -5 + -7 +...
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