Question:

The product of first nine terms of a GP is, in general, equal to which one of the following?

Updated On: Jul 7, 2022
  • The 9th power of the 4th term
  • The 4th power of the 9th term
  • The 5th power of the 9th term
  • The 9th power of the 5th term
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The Correct Option is D

Solution and Explanation

Let a be the first term and r, the common ratio First nine terms of a GP are $a, ar, ar^2, .... ar^8$. $\therefore \, P = a.ar. ar^2 .... ar^8$ $= a^9.r^{1+2+...+8} $ $= a^9.r^{\frac{8.9}{2}}= a^9r^{36}$ $= (ar^4)^9 = (T_5)^9$ = 9th power of the 5th term
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Concepts Used:

Geometric Progression

What is Geometric Sequence?

A geometric progression is the sequence, in which each term is varied by another by a common ratio. The next term of the sequence is produced when we multiply a constant to the previous term. It is represented by: a, ar1, ar2, ar3, ar4, and so on.

Properties of Geometric Progression (GP)

Important properties of GP are as follows:

  • Three non-zero terms a, b, c are in GP if  b2 = ac
  • In a GP,
    Three consecutive terms are as a/r, a, ar
    Four consecutive terms are as a/r3, a/r, ar, ar3
  • In a finite GP, the product of the terms equidistant from the beginning and the end term is the same that means, t1.tn = t2.tn-1 = t3.tn-2 = …..
  • If each term of a GP is multiplied or divided by a non-zero constant, then the resulting sequence is also a GP with a common ratio
  • The product and quotient of two GP’s is again a GP
  • If each term of a GP is raised to power by the same non-zero quantity, the resultant sequence is also a GP.

If a1, a2, a3,… is a GP of positive terms then log a1, log a2, log a3,… is an AP (arithmetic progression) and vice versa