an=a1q^(n-1)
Sn = a1+a2+...+an
= a1(q^n-1)/(q-1)
Tn = a1.a2.a3.....an
= (a1)^n ( q.q^2...q^(n-1) )
=(a1)^n . q^[n(n-1)/2]
bn = b1+(n-1)d
Sn =b1+b2+...+bn = (2b1+(n-1)d)n/2
Tn = b1.b2....bn
= b1(b1+d)(b1+2d)...(b1+(n-1)d)
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