Unseen errors

Algebra Level 3

For the function f ( x ) = x 5 + 1 f(x)={x}^5+1 , find the number of real values of x x such that f ( x ) = f 1 ( x ) f(x)={f}^{-1}(x) .


The answer is 1.

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3 solutions

Gaurav Jain
Jul 6, 2015

The no of solutions of the equation f ( x ) = f 1 ( x ) f(x)= f{-1}(x) should occur for the ordered pairs of type ( a , a ) algebrically which gives us the equation
( x 5 x + 1 = 0 ) ({ x }^{ 5 }-x+1=0 ) which can have at most 5 real roots or at least 1. (think why ). Geometrically it is the point where the three curves f ( x ) , f 1 ( x ) f(x) , f{-1}(x) and x=y do meet together. To check whether f(x) has 5 ,3 or 1 real root we proceed as: f ( x ) = x 5 x + 1 f(x)={ x }^{ 5 }-x+1
d d x f ( x ) = 5 x 4 1 \frac { d }{ dx } f(x)=5{ x }^{ 4 }-1 so , d d x f ( x ) = 0 \frac { d }{ dx } f(x)=0 at x = ± 1 5 1 / 4 x=\pm \frac { 1 }{ { 5 }^{ 1/4 } } but we check for both values of x , f(x)>0. before and after the value range of x ϵ ( 1 5 1 / 4 , 1 5 1 / 4 ) x\epsilon (-\frac { 1 }{ { 5 }^{ 1/4 } } ,\frac { 1 }{ { 5 }^{ 1/4 } } ) it is monotonically increasing or decreasing and prior to the decreasing range there must be a root but for x > 1 5 1 / 4 x\quad >-\frac { 1 }{ { 5 }^{ 1/4 } } as we see it is always positive. Therefore it ends to have a singe root and further only a single solution for the equation.

@Calvin Lin Sir please help! Latex signs are not clear here !!

Gaurav Jain - 5 years, 11 months ago

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You need to put your equations in \ ( \ ) \backslash ( \quad \backslash) , and not /( /). I've edited the first 2 equations for your reference.

Calvin Lin Staff - 5 years, 11 months ago

Nice solution ! Upvoted!

Rohit Ner - 5 years, 11 months ago

f ( x ) = f 1 ( x ) f(x)=f^{-1}(x) can be solved by graphing. Where the graph of f ( x ) f(x) is known and its inverse function will be the reflection of f f with y = x y=x . without graphing we can realize that there is only one intersection with y = x y=x and therefore with f 1 f^{-1}

Sammy Berger
Jul 3, 2015

This problem takes a few steps. First, find f^{-1}(x). To do so, take the equation y = x^5 + 1, switch y and x, then solve for y. Set that equation equal to the original one, and then solve for x. I put it into Wolfram Alpha once it got to that point, in all honesty. It turns out there is just 1 real solution.

Moderator note:

This is not a proper solution. Can you solve this problem without using computational device?

Any one can use wolfram alpha..you haven't made any efforts to solve the problem.

monty g - 5 years, 11 months ago

May be draw a rough sketch of it... i mean GRAPH it..

Sriram Vudayagiri - 5 years, 11 months ago

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