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r=a4-a3
r=19-7=12
a1+2*12=7
a1=7-24
a1=-17
an=a1+r(n-1)
an=-17+12(n-1)=-17+12n-12
an=2n-29
3.
a)
a10=5+1,5*9=5+13,5=18,5
b)
Sn=1/2*(a1+an)*n
S20=1/2*(-32+121)*20=10*89=890