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a2=-7
r=a2-a1=-7-(-5)=-2
an=-31
an=a1+(n-1)r
-31=-5+(n-1)*(-2)
-31+5=-2n+2
2n=2+26
n=28/2
n=14
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S14=[a1+a14]/2 *14=[-5-31]*7=-252