If f is a function satisfying | Class 11 Mathematics Chapter Sequence and Series, Sequence and Series NCERT Solutions

Question:

If is a function satisfying f(x +y) = f(x) f(y) for all x,y element ofN  such that f(1) = 3

sum from x equals 1 to n of space f left parenthesis x right parenthesis space equals space 120 and  , find the value of n.

Answer:

It is given that,

(x + y) = (x) × (y) for all xy ∈ N … (1)

(1) = 3

Taking x = y = 1 in (1), we obtain

f (1 + 1) = (2) = (1) (1) = 3 × 3 = 9

Similarly,

(1 + 1 + 1) = (3) = (1 + 2) = (1) (2) = 3 × 9 = 27

(4) = (1 + 3) = f (1) (3) = 3 × 27 = 81

∴ (1), (2), (3), …, that is 3, 9, 27, …, forms a G.P. with both the first term and common ratio equal to 3.

It is known that, S subscript n space equals space fraction numerator a open parentheses r to the power of n space minus space 1 close parentheses over denominator r space minus 1 end fraction

It is given that, sum from x equals 1 to n of space f left parenthesis x right parenthesis space equals space 120

therefore space 120 space equals space fraction numerator 3 open parentheses 3 to the power of n space minus 1 close parentheses over denominator 3 minus 1 end fraction
rightwards double arrow 120 space equals space 3 over 2 open parentheses 3 to the power of n space minus space 1 close parentheses
rightwards double arrow 3 to the power of n space minus space 1 space equals space 80
rightwards double arrow 3 to the power of n space equals space 81 space equals space 3 to the power of 4 space
therefore space n space equals 4

Thus, the value of n is 4.


Study Tips for Answering NCERT Questions:

NCERT questions are designed to test your understanding of the concepts and theories discussed in the chapter. Here are some tips to help you answer NCERT questions effectively:

  • Read the question carefully and focus on the core concept being asked.
  • Reference examples and data from the chapter when answering questions about Sequence and Series.
  • Review previous year question papers to get an idea of how such questions may be framed in exams.
  • Practice answering questions within the time limit to improve your speed and accuracy.
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Comment(s) on this Question

Welcome to the NCERT Solutions for Class 11 Mathematics - Chapter . This page offers a step-by-step solution to the specific question from Excercise 5 , Question 7: If f is a function satisfying f(x +y) = f(x) f(y) for all x,y N  such that f(1) ....