When two lines meet!! 4

Algebra Level 4

Find the number of roots of the equation f ( x ) = log 10 x cos ( sin ( x ) ) f(x) =\log_{10}\sqrt{|x|} - \cos(\sin(x)) for 31 π x 31 π -31\pi \le x \le 31\pi .

Hint: 1 0 2 cos 1 = 12 10^{2\cos 1} = 12


Inspiration Aniket Sanghi


Bonus: Find total number of roots for x R x\in R

All of my problems are original


The answer is 108.

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

1 solution

Aryan Sanghi
May 29, 2020

For start, let's consider graph of each


Graph of l o g 10 x log_{10}\sqrt{|x|}

Graph of c o s ( s i n ( x ) ) cos(sin(x))


So, we can see that c o s ( s i n ( x ) ) [ c o s ( 1 ) , 1 ] cos(sin(x))\in [cos(1), 1]

So, we have to find for what values of x x , l o g 10 x [ c o s ( 1 ) , 1 ] log_{10}\sqrt{|x|} \in [cos(1), 1]

c o s ( 1 ) l o g 10 x 1 cos(1) \leq log_{10}\sqrt{|x|} \leq 1

1 0 c o s ( 1 ) x 10 10^{cos(1)} \leq \sqrt{|x|} \leq 10

1 0 2 c o s ( 1 ) x 100 10^{2cos(1)} \leq |x| \leq 100

12 x 100 12 \leq |x| \leq 100

4 π x 31.83 π 4\pi \leq |x| \leq 31.83\pi

As it is mentioned for x 31 π |x| \leq 31π

4 π x 31 π 4\pi \leq |x| \leq 31\pi

31 π x 4 π or 4 π x 31 π . . . . . . . . . . . ( 1 ) -31\pi \leq x \leq -4\pi \text{ or } 4\pi \leq x \leq 31\pi ........... (1)


Now, for each x [ n π , ( n + 1 ) π ] x \in [n\pi, (n+1)\pi] in ( 1 ) (1) , there are 2 \boxed{2} intersections

So, there are a total of ( ( 31 4 ) × 2 ) × 2 = 108 \color{#3D99F6}{\boxed{((31 - 4) × 2) × 2 = 108}} intersections


Here is a graph combining both equations

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...