ciara180
ciara180 ciara180
  • 03-08-2018
  • Mathematics
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Find S15 for the series -1 + -3 + -5 + -7 +...

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sarajevo
sarajevo sarajevo
  • 03-08-2018

The given series is [tex] -1 + -3 + -5 + -7 +... [/tex]

Alternatively it can be written as , say s then

[tex] 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\\ [/tex]

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