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un=a+(n-1)b
58=3 +(n-1)5
n-1= 11 ... n=12
s12=1/2n(a+Un)=1/2x 12 x(3+58)= 366
a=30 b -5Un = -40
-40=30 +(n-1)(-5)
-70/(-5)=n-1 ...n=15
Sn=1/2x15(30-40)= - 75