Question:

Let $f:(-1,1)\rightarrow$ R be such that $f(cos 4 \theta)=\frac{2}{2-sec^2 \theta}$ for $\theta \in \Bigg(0,\frac{\pi}{4}\Bigg)\cup\Bigg(\frac{\pi}{4},\frac{\pi}{2}\Bigg).$ Then, the value(s) of $f\Bigg(\frac{1}{3}\Bigg)$ is are

Updated On: Jun 23, 2023
  • $1-\sqrt{\frac{3}{2}}$
  • $1+\sqrt{\frac{3}{2}}$
  • $1-\sqrt{\frac{2}{3}}$
  • $1+\sqrt{\frac{2}{3}}$
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The Correct Option is B

Solution and Explanation

$f(cos 4\theta)=\frac{2}{2-sec^2 \theta}\hspace15mm ...(i)$ At $\hspace15mm cos4\theta=\frac{1}{3} \Rightarrow 2 cos^2 \theta-1=\frac{1}{3}$ $\Rightarrow \hspace15mm cos^22\theta=\frac{2}{3}\Rightarrow cos 2\theta=\pm\sqrt{\frac{2}{3}}\hspace15mm ...(ii)$ $\therefore \hspace15mm f(cos4\theta)=\frac{2.cos^2 \theta}{2 cos^2 \theta-1}= frac{1+cos^\theta}{cos 2\theta}$ $\Rightarrow \hspace15mm f\Big(\frac{1}{3}\Big)=1\pm\sqrt{\frac{3}{2}}\hspace15mm [from E(ii)]$
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Concepts Used:

Functions

A function is a relation between a set of inputs and a set of permissible outputs with the property that each input is related to exactly one output. Let A & B be any two non-empty sets, mapping from A to B will be a function only when every element in set A has one end only one image in set B.

Kinds of Functions

The different types of functions are - 

One to One Function: When elements of set A have a separate component of set B, we can determine that it is a one-to-one function. Besides, you can also call it injective.

Many to One Function: As the name suggests, here more than two elements in set A are mapped with one element in set B.

Moreover, if it happens that all the elements in set B have pre-images in set A, it is called an onto function or surjective function.

Also, if a function is both one-to-one and onto function, it is known as a bijective. This means, that all the elements of A are mapped with separate elements in B, and A holds a pre-image of elements of B.

Read More: Relations and Functions