Answer : ‘Tn’ represents the n^th term of a G.P. series. Tn = ar^n-1

⇒486 = a(3)n-1

⇒486 = a( 3n ÷ 3) )

⇒486 × 3 = a(3n)

⇒1458 = a(3n ) ………(i)

Sum of a G.P. series is represented by the formula,  when r≠1. ‘Sn’ represents the sum of the G.P. series upto nth terms, ‘a’ represents the first term, ‘r’ represents the common ratio and ‘n’ represents the number of terms.

⇒728 × 2 = a (3n)-a …… [Putting a(3n ) = 1458 from (i)]

⇒1456 = 1458 -a

⇒1456-1458 = -a

⇒-2=-a [Multipying both sides by -1]

⇒a = 2