The Total solution for NCERT class 6-12
Given:
Sum of GP = 728
Where, r = 3, a = ?
Firstly,
Tn =arn-1
486 = a3n-1
486 = a3n/3
486 (3) = a3n
1458 = a3n ….Equation (i)
By using the formula,
Sum of GP for n terms= a(rn – 1 )/(r – 1)
728 = a (3n –1)/2
1456 = a3n –a … equation (2)
Subtracting equation(1) from (2) we get
1458 – 1456 = a.3n –a.3n + a
a = 2.
∴ The first termis 2