a1=d
an=a1+(n-1)d=nd
sn=na1+n(n-1)d/2=(n+1)nd/2
s20=(20+1)*20*d/2=210d=840
d=4
a1=4
an=4n
2.
sn=(n+1)nd/2=2(n+1)n=84
(n+1)n=42
n=6
an = nd
S20=840
210d =840
d= 4
an = 4n
Sn = 84
(4n+4)n/2 = 84
n^2+n-42=0
(n-7)(n+6)=0
n=7
S7=84
a1=d
an=a1+(n-1)d=nd
sn=na1+(n-1)nd/2=(n+1)nd/2
s20=(20+1)*20*d/2=840 d=4
an=4n
sn=2n(n+1)=84 n=6
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