Question 626886
You're subtracting 5 each time. The first 7 terms are


12+7+2+(-3)+(-8)+(-13)+(-18)


The nth term is 12+(n-1)*(-5) which becomes -5n+17


So the sum of the first 7 terms using sigma notation is 


<img src="http://latex.codecogs.com/gif.latex?\LARGE \sum_{k=1}^{7}-5k+17" title="x" />

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