All Exams Test series for 1 year @ ₹349 only
Question

Let $f$ be a polynomial function such that $\log_2 (f(x)) = \left( \log_2 \left( 2 + \frac{2}{3} + \frac{2}{9} + \dots \infty \right) \right) \cdot \log_3 \left( 1 + \frac{f(x)}{f\left(\frac{1}{x}\right)} \right), x > 0$ and $f(6) = 37$. Then $\sum_{n=1}^{10} f(n)$ is equal to ________.

Geometric Series Sum Calculation

The expression inside the first logarithm is a geometric series: $2 + \frac{2}{3} + \frac{2}{9} + \dots \infty$ This series has a first term $a = 2$ and a common ratio $r = \frac{1}{3}$. The sum to infinity ($S$) is calculated as: $S = \frac{a}{1-r} = \frac{2}{1 - \frac{1}{3}} = \frac{2}{\frac{2}{3}} = 3$. Therefore, $\log_2 \left( 2 + \frac{2}{3} + \frac{2}{9} + \dots \infty \right) = \log_2(3)$.

Logarithmic Equation Simplification

The given equation is: $\log_2 (f(x)) = \left( \log_2(3) \right) \cdot \log_3 \left( 1 + \frac{f(x)}{f\left(\frac{1}{x}\right)} \right)$ Using the change of base formula for logarithms, $\log_a b = \frac{\log_c b}{\log_c a}$, we can write $\log_3 \left( 1 + \frac{f(x)}{f\left(\frac{1}{x}\right)} \right) = \frac{\log_2 \left( 1 + \frac{f(x)}{f\left(\frac{1}{x}\right)} \right)}{\log_2(3)}$. Substituting this back into the equation gives: $\log_2 (f(x)) = \log_2(3) \cdot \frac{\log_2 \left( 1 + \frac{f(x)}{f\left(\frac{1}{x}\right)} \right)}{\log_2(3)}$ $\log_2 (f(x)) = \log_2 \left( 1 + \frac{f(x)}{f\left(\frac{1}{x}\right)} \right)$ Since the logarithm function is one-to-one, we can equate the arguments: $f(x) = 1 + \frac{f(x)}{f\left(\frac{1}{x}\right)}$ Rearranging this equation (assuming $f(x) \neq 0$): $f(x) - 1 = \frac{f(x)}{f\left(\frac{1}{x}\right)}$ Dividing by $f(x)$: $1 - \frac{1}{f(x)} = \frac{1}{f\left(\frac{1}{x}\right)}$ $1 = \frac{1}{f(x)} + \frac{1}{f\left(\frac{1}{x}\right)}$

Polynomial Function Identification

We are given that $f$ is a polynomial function. Let's test a potential polynomial form that satisfies the derived functional equation $1 = \frac{1}{f(x)} + \frac{1}{f\left(\frac{1}{x}\right)}$. Consider a polynomial of the form $f(x) = 1+x^k$ for some non-negative integer $k$ (since $f$ is a polynomial). Then $f\left(\frac{1}{x}\right) = 1 + \left(\frac{1}{x}\right)^k = 1 + x^{-k}$. Substituting into the equation: $\frac{1}{f(x)} + \frac{1}{f\left(\frac{1}{x}\right)} = \frac{1}{1+x^k} + \frac{1}{1+x^{-k}} = \frac{1}{1+x^k} + \frac{1}{\frac{x^k+1}{x^k}} = \frac{1}{1+x^k} + \frac{x^k}{1+x^k} = \frac{1+x^k}{1+x^k} = 1$. This form $f(x) = 1+x^k$ satisfies the functional equation.

Now, we use the condition $f(6) = 37$: $f(6) = 1+6^k = 37$ $6^k = 37 - 1$ $6^k = 36$ $6^k = 6^2$ This implies $k=2$. So, the polynomial function is $f(x) = 1+x^2$. This is a valid polynomial function.

Final Summation Calculation

We need to calculate $\sum_{n=1}^{10} f(n)$. Substituting $f(x) = 1+x^2$: $\sum_{n=1}^{10} f(n) = \sum_{n=1}^{10} (1+n^2)$ This sum can be split into two parts: $= \sum_{n=1}^{10} 1 + \sum_{n=1}^{10} n^2$ The sum of the constant term is: $\sum_{n=1}^{10} 1 = 10 \times 1 = 10$. The sum of the first 10 squares is calculated using the formula $\sum_{n=1}^{N} n^2 = \frac{N(N+1)(2N+1)}{6}$. For $N=10$: $\sum_{n=1}^{10} n^2 = \frac{10(10+1)(2 \cdot 10 + 1)}{6} = \frac{10 \cdot 11 \cdot 21}{6} = \frac{2310}{6} = 385$. Adding the two parts together: Total sum $= 10 + 385 = 395$.

Was this answer helpful?

Similar Questions

  1. Let $(2\alpha, \alpha)$ be the largest interval in which the function $f(t) = \frac{|t+1|}{t^2}, t < 0$, is strictly decreasing. Then the local maximum value of the function $g(x) = 2\log_e(x - 2) + \alpha x^2 + 4x - \alpha$, $x > 2$, is ______
  2. Let $f: \mathbb{R} \rightarrow \mathbb{R}$ be a twice differentiable function such that the quadratic equation $f(x)m^2 - 2f'(x)m + f''(x) = 0$ in m, has two equal roots for every $x \in \mathbb{R}$. If $f(0) = 1, f'(0) = 2$, and $(\alpha, \beta)$ is the largest interval in which the function $f(\log_e x - x)$ is increasing, then $\alpha + \beta$ is equal to ____________.
  3. The number of points in the interval $[2, 4]$, at which the function $f(x) = \left[ x^2 - x - \frac{1}{2} \right]$, where $[\cdot]$ denotes the greatest integer function, is discontinuous, is __________.
  4. The number of points of discontinuity of the function $f (x) = \left[\frac{x^2}{2}\right] - [\sqrt{x}], x \in [0,4]$, where $[.]$denotes the greatest integer function, is ________
  5. The number of functions $f: \{1, 2, 3, 4\} \rightarrow \{a, b, c\}$, which are not onto, is:
  6. If $y = \tan^{-1}\left(\frac{3\cos x - 4\sin x}{4\cos x + 3\sin x}\right) + 2\tan^{-1}\left(\frac{x}{1 + \sqrt{1 - x^2}}\right)$, then $\frac{dy}{dx}$ at $x = \frac{\sqrt{3}}{2}$ is equal to:
  7. Let $f(x) = \lim_{\theta \rightarrow 0} \left(\frac{\cos\pi x - x^{\left(\frac{2}{\theta}\right)}\sin(x - 1)}{1 + x^{\left(\frac{2}{\theta}\right)}(x - 1)}\right)$, $x \in \mathbf{R}$. Consider the following two statements :
    (I) $f(x)$ is discontinous at $x = 1$.
    (II) $f(x)$ is continous at $x = -1$.
    Then,
  8. The sum of all the elements in the range of $f(x) = \text{Sgn}(\sin x) + \text{Sgn}(\cos x) + \text{Sgn}(\tan x) + \text{Sgn}(\cot x)$,
    $x \neq \frac{n\pi}{2}, n \in \mathbf{Z}$, where $\text{Sgn}(t) = \begin{cases} 1, & \text{if } t > 0 \\ -1, & \text{if } t < 0 \end{cases}$, is :
  9. If $f(x) = \begin{cases} \frac{a|x| + x^2 - 2(\sin|x|)(\cos|x|)}{x} & , \ x \neq 0 \\ b & , \ x = 0 \end{cases}$ is continuous at $x = 0$, then $a + b$ is equal to
  10. If the function $f(x) = \frac{e^x\left(e^{\tan x - x} - 1\right) + \log_e(\sec x + \tan x) - x}{\tan x - x}$ is continuous at $x = 0$, then the value of $f(0)$ is equal to

Important Questions from Differential Calculus

  1. Let $(2\alpha, \alpha)$ be the largest interval in which the function $f(t) = \frac{|t+1|}{t^2}, t < 0$, is strictly decreasing. Then the local maximum value of the function $g(x) = 2\log_e(x - 2) + \alpha x^2 + 4x - \alpha$, $x > 2$, is ______
  2. Let $f: \mathbb{R} \rightarrow \mathbb{R}$ be a twice differentiable function such that the quadratic equation $f(x)m^2 - 2f'(x)m + f''(x) = 0$ in m, has two equal roots for every $x \in \mathbb{R}$. If $f(0) = 1, f'(0) = 2$, and $(\alpha, \beta)$ is the largest interval in which the function $f(\log_e x - x)$ is increasing, then $\alpha + \beta$ is equal to ____________.
  3. For a real number $y$, consider $[y]$ denotes the greatest integer less than or equal to $y$. 
    If $f(x) = \frac{\tan(\pi[x-\pi])}{1+[x]^2}$, then

  4. The number of points in the interval $[2, 4]$, at which the function $f(x) = \left[ x^2 - x - \frac{1}{2} \right]$, where $[\cdot]$ denotes the greatest integer function, is discontinuous, is __________.
  5. The number of points of discontinuity of the function $f (x) = \left[\frac{x^2}{2}\right] - [\sqrt{x}], x \in [0,4]$, where $[.]$denotes the greatest integer function, is ________
Need Expert Advice?
More Questions from JEE Main

Start Your Preparation with Prepp Mobile App

Download the app from Google Play & App Store
Download the app from Google Play & App Store
Prepp Mobile App