How will this be done?

Calculus Level 4

A monic polynomial function p ( x ) p(x) of degree 5 increases in the interval x < 1 x<1 and x > 3 x>3 and decreases in the interval 1 < x < 3 1<x<3 . Call f ( x ) f(x) the polynomial obtained by differentiating p ( x ) p(x) once with respect to x x .

Given that p ( 0 ) = 4 p(0) = 4 and f ( 2 ) = 0 f(2) = 0 . Calculate the value of f ( 4 ) f(4) .

If you think that the data is insufficient, then submit 1 as your answer.


The answer is 60.

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1 solution

Pranav Rao
Apr 25, 2016

It's given that p(x) increases for x<1, and for x>3 and it decreases for 1<x<3. So f(x) is positive before x =1 and negative after 1. As f(x) is continuous, f(1) is 0. Similarly f(3)=0. Now we know that f(2) is also 0. But we know that f(x) is non positive for (1,3) so we can say that f(x) touches the X axis at 2. Now as f (x) is 4 degree polynomial with leading coefficient 5, and has factors x-1 and x-3 and (x-2)² we get f(x) as 5(x-1)(x-2)²(x-3).

Exactly!! upvoted +1!

Prakhar Bindal - 5 years, 1 month ago

How do you know that f f cannot have a root a ( 1 , 3 ) a\in (1,3) , other than 2 2 ?

Samrat Mukhopadhyay - 4 years, 9 months ago

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Because then the condition that p(x) increases in the interval x<1 and x>3 wont be satisfied you may check yourself

Prakhar Bindal - 4 years, 9 months ago

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Ok, yeah, it happens because of the degree 5 5 , got it.

Samrat Mukhopadhyay - 4 years, 9 months ago

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@Samrat Mukhopadhyay are you from IIT Kharagpur?

Prakhar Bindal - 4 years, 8 months ago

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@Prakhar Bindal Yes, I am a research scholar there.

Samrat Mukhopadhyay - 4 years, 8 months ago

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@Samrat Mukhopadhyay I Also Aspire to get into one of the IIT's this year by appearing in JEE Advanced 2017

Prakhar Bindal - 4 years, 8 months ago

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@Prakhar Bindal Best of you! (Its better to rely on yourself than luck! :-) )

Samrat Mukhopadhyay - 4 years, 8 months ago

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@Samrat Mukhopadhyay Yeah! very true

Prakhar Bindal - 4 years, 8 months ago

Smh Found out the roots are 1,3,2(repeated root as 2 is a point of inflection) But forgot the factor of 5 Hence entered 12 :P

Suhas Sheikh - 2 years, 10 months ago

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