Sn=59,5
a1=-3,5
a2=-2
d=a2-a1=-2-(-3,5)=-2+3,5=1,5
an=a1+d*(n-1)=-3,5+1,5*(n-1)=-3,5+1,5n-1,5=-5+1,5n
Sn=(a1+an)*n/2
59,5=(-3,5-5+1,5n)/2
-8,5+1,5n=59,5*2
1,5n=119+8,5
1,5n=127,5
n=127,5/1,5=1275/15=85
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Answers & Comments
Sn=59,5
a1=-3,5
a2=-2
d=a2-a1=-2-(-3,5)=-2+3,5=1,5
an=a1+d*(n-1)=-3,5+1,5*(n-1)=-3,5+1,5n-1,5=-5+1,5n
Sn=(a1+an)*n/2
59,5=(-3,5-5+1,5n)/2
-8,5+1,5n=59,5*2
1,5n=119+8,5
1,5n=127,5
n=127,5/1,5=1275/15=85