Найдём а2:
Sn =((a1+an)/2)*n
S3=((8+18)/2)*3 =39
39= a1+a2+a3
a2= 39-8-18=13
Sn=((2*a1+b(n-1))/2)*n
b=a2-a1=13-8=5
S4=((2*8+5*3)/2)*4 = 62
От: 62
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Verified answer
Найдём а2:
Sn =((a1+an)/2)*n
S3=((8+18)/2)*3 =39
39= a1+a2+a3
a2= 39-8-18=13
Sn=((2*a1+b(n-1))/2)*n
b=a2-a1=13-8=5
S4=((2*8+5*3)/2)*4 = 62
От: 62