When two lines meet! 5

Algebra Level 4

Find the number of roots of the equation f ( x ) = tan x 2 x f(x) = \tan x - 2x for 31 π 2 x 31 π 2 -\frac{31\pi}{2} \leq x \leq \frac{31\pi}{2} .


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30 36 32 34 29 33 35 31

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1 solution

Aryan Sanghi
Jun 6, 2020

Above question can be reduced to

Find number of points of intersection of graphs of tan x and 2x

Here is the graph of tan x and 2x superimposed


Now, let's prove there will be 3 intersections on the range x [ π 2 , π 2 ] x \in [-\frac{\pi}{2}, \frac{\pi}{2}]

Now, it is evident that at x = 0 , t a n x = 2 x = 0 x = 0, tan x = 2x = 0 , so there will be an intersection at x = 0 x = 0

After that, let f ( x ) = t a n x and g ( x ) = 2 x \text{let } f(x) = tan x \text{ and } g(x) = 2x

f ( x ) = s e c ² x and g ( x ) = 2 f'(x) = sec²x \text{ and } g'(x) = 2

So, at x = 0 , f ( x ) < g ( x ) \text{So, at } x = 0, f'(x) \lt g'(x)

But, for x π 2 , f ( x ) g ( x ) \text{But, for } x \geq \frac{\pi}{2}, f'(x) \geq g'(x)

So, there will be another intersection for x ( 0 , π 2 ] x \in (0, \frac{\pi}{2}]

Similarly, it can be proved that there will be another intersection for x [ π 2 , 0 ) x \in [-\frac{\pi}{2}, 0)

So, there are three intersections for x [ π 2 , π 2 ] x \in [-\frac{\pi}{2}, \frac{\pi}{2}]


Now, for all other x [ ( 2 k 1 ) π 2 , ( 2 k + 1 ) π 2 ] x \in [\frac{(2k - 1)\pi}{2}, \frac{(2k + 1)\pi}{2}] where k I [ 0 ] k \in I - [0] , there is 1 intersection

So, total intersections are n = 2 ( 15 ) + 3 n = 2(15) + 3

n = 33 \color{#3D99F6}{\boxed{n = 33}}

I am pretty sure I attempted this question.

Did you re-post it?

@Aryan Sanghi

A Former Brilliant Member - 11 months, 4 weeks ago

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No, just changed its topic twice. Sorry.

Aryan Sanghi - 11 months, 4 weeks ago

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Well, I already attempted this question. So...

A Former Brilliant Member - 11 months, 4 weeks ago

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@A Former Brilliant Member Nothing is changed actually. You've still solved it.

Aryan Sanghi - 11 months, 4 weeks ago

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@Aryan Sanghi I got it incorrect twice!

A Former Brilliant Member - 11 months, 4 weeks ago

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@A Former Brilliant Member No, just once! I haven't reposted it. Just changed the topic.

Aryan Sanghi - 11 months, 4 weeks ago

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@Aryan Sanghi So, doesn't that mean my original answer should be here? My original answer was 31 31 ...

A Former Brilliant Member - 11 months, 4 weeks ago

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@A Former Brilliant Member So isn't it here?

Aryan Sanghi - 11 months, 4 weeks ago

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@Aryan Sanghi No, it wasn't there. I had to attempt it again.

A Former Brilliant Member - 11 months, 4 weeks ago

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@A Former Brilliant Member But I can see that you did attempt it. No worries anyway, it won't affect your level twice.

Aryan Sanghi - 11 months, 4 weeks ago

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@Aryan Sanghi Does that mean it was a bug?

A Former Brilliant Member - 11 months, 4 weeks ago

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@A Former Brilliant Member Yes, I guess it was. Try mailing them to support@brilliant.org

Aryan Sanghi - 11 months, 4 weeks ago

@Brilliant Mathematics , I think I have a bug. Look at the conversation above between @Aryan Sanghi and me to understand.

A Former Brilliant Member - 11 months, 4 weeks ago

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Hi Yajat, the system shows that you made one attempt (of 36) once.

It is impossible to redo a community problem after you have used all your attempts. In this case, this problem is an MCQ (Multiple Choice Question), so our system only allows you to submit one attempt.

Brilliant Mathematics Staff - 11 months, 3 weeks ago

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I am trying to say before, last month, I believe, I answered 31 31 . Then @Aryan Sanghi changed the topic twice. Doesn't that mean that the answer should stay the same? @Brilliant Mathematics

A Former Brilliant Member - 11 months, 3 weeks ago

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@A Former Brilliant Member Hi, the system shows that you have viewed this for the first time on June 19 2020, and submitted an attempt (of 36) 16 seconds later.

Brilliant Mathematics Staff - 11 months, 3 weeks ago

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@Brilliant Mathematics Well, I definitely know I attempted it before.

Thanks for the help, @Brilliant Mathematics

A Former Brilliant Member - 11 months, 3 weeks ago

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