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a1=1
r=4-1=3
an=a1+r(n-1)
1+3(n-1)=28
3n-3+1=28
3n=28+2
n=10
Sn=1/2*(1+28)*10=5*29=145
b)
a1=1
r=-5-1=-6
an=-119
1-6(n-1)=-119
-6n+6+1=-119
-6n=-119-7
-6n=-126
n=21
Sn=1/2(1-119)*21=-118*1/2*21=-59*21=-1239