1.

How many terms of the series 2 + 6 + 18 + …. Must be taken to make the sum equal to 728?

Answer»

Given: 

Sum of GP = 728 

∴ Common Ratio = r = \(\frac{6}{2}\) = 3

 a = 2

 To find: Number of terms = n. 

Sum of GP for n terms = \(\frac{a(r^n-1)}{r-1}\)

⇒ 728 = \(\cfrac{2(3^n -1)}{3-1}\)

⇒ 728 = 3n - 1 

⇒ 729 = 3n 

⇒ 36 = 3

∴ n = 6.



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