Find the number of roots of the equation f ( x ) = tan x − 2 x for − 2 3 1 π ≤ x ≤ 2 3 1 π .
Inspiration Aniket Sanghi
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I am pretty sure I attempted this question.
Did you re-post it?
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No, just changed its topic twice. Sorry.
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Well, I already attempted this question. So...
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@A Former Brilliant Member – Nothing is changed actually. You've still solved it.
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@Aryan Sanghi – I got it incorrect twice!
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@A Former Brilliant Member – No, just once! I haven't reposted it. Just changed the topic.
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@Aryan Sanghi – So, doesn't that mean my original answer should be here? My original answer was 3 1 ...
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@A Former Brilliant Member – So isn't it here?
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@Aryan Sanghi – No, it wasn't there. I had to attempt it again.
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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.
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@Aryan Sanghi – Does that mean it was a bug?
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@A Former Brilliant Member – Yes, I guess it was. Try mailing them to support@brilliant.org
@Brilliant Mathematics , I think I have a bug. Look at the conversation above between @Aryan Sanghi and me to understand.
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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.
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I am trying to say before, last month, I believe, I answered 3 1 . Then @Aryan Sanghi changed the topic twice. Doesn't that mean that the answer should stay the same? @Brilliant Mathematics
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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.
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@Brilliant Mathematics – Well, I definitely know I attempted it before.
Thanks for the help, @Brilliant Mathematics
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Above question can be reduced to
Here is the graph of tan x and 2x superimposed
Now, let's prove there will be 3 intersections on the range x ∈ [ − 2 π , 2 π ]
Now, it is evident that at x = 0 , t a n x = 2 x = 0 , so there will be an intersection at x = 0
After that, let f ( x ) = t a n x and g ( x ) = 2 x
f ′ ( x ) = s e c ² x and g ′ ( x ) = 2
So, at x = 0 , f ′ ( x ) < g ′ ( x )
But, for x ≥ 2 π , f ′ ( x ) ≥ g ′ ( x )
So, there will be another intersection for x ∈ ( 0 , 2 π ]
Similarly, it can be proved that there will be another intersection for x ∈ [ − 2 π , 0 )
So, there are three intersections for x ∈ [ − 2 π , 2 π ]
Now, for all other x ∈ [ 2 ( 2 k − 1 ) π , 2 ( 2 k + 1 ) π ] where k ∈ I − [ 0 ] , there is 1 intersection
So, total intersections are n = 2 ( 1 5 ) + 3
n = 3 3