The common ratio of a G.P. is 3 and the last term is 486. If the sum of these terms be 728, find the first term.
r=3, last term (an) is 486
Sum of terms =Sn=728, a=?
an=486
⇒arn−1=486
⇒a(3)n−1=486
⇒a(3)n=486×3
⇒a(3)n=1458 ..........(i)
Now Sn=728
⇒728=a(3n−13−1)
⇒728={a(3)n−a2}
⇒1456=a(3)n
⇒1456=1458−a [From (i)]
⇒a=2