Calculus + Algebra = Calgebra!

Calculus Level 4

Let f ( x ) f(x) be a monic quartic polynomial such that ,

f ( 1 ) = 11 f(1) = 11 , f ( 2 ) = 39 f(2) = 39 , f ( 3 ) = 123 f(3) = 123 , f ( 4 ) = 323 f(4) = 323

Then evaluate : lim x 1 ( f ( x ) . f ( x ) ) \lim _{x\rightarrow 1}\left ( f^{'}(x) . f^{''}(x)\right )


The answer is 252.

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1 solution

Rishabh Jain
Jan 24, 2016

f ( 1 ) = 11 = ( 1 2 + 1 ) 2 + ( 1 + 5 ) ( 1 1 ) + 7 f ( 2 ) = 23 = ( 2 2 + 1 ) 2 + ( 2 + 5 ) ( 2 1 ) + 7 f ( 3 ) = 123 = ( 3 2 + 1 ) 2 + ( 3 + 5 ) ( 3 1 ) + 7 f ( 4 ) = 323 = ( 4 2 + 1 ) 2 + ( 4 + 5 ) ( 4 1 ) + 7 \small{f(1)=11=(\color{#3D99F6}{1}^2+1)^2+(\color{#3D99F6}{1}+5)(\color{#3D99F6}{1}-1)+7\\ f(2)=23=(\color{#3D99F6}{2}^2+1)^2+(\color{#3D99F6}{{2}}+5)(\color{#3D99F6}{2}-1)+7\\ f(3)=123=(\color{#3D99F6}{3}^2+1)^2+(\color{#3D99F6}{3}+5)(\color{#3D99F6}{3}-1)+7\\ f(4)=323=(\color{#3D99F6}{4}^2+1)^2+(\color{#3D99F6}{4}+5)(\color{#3D99F6}{4}-1)+7\\} H e n c e , f ( x ) = ( x 2 + 1 ) 2 + ( x + 5 ) ( x 1 ) + 7 = x 4 + 3 x 2 + 4 x + 3 Hence, \Large f(x)=(x^2+1)^2+(x+5)(x-1)+7\\=x^4+3x^2+4x+3 f ( x ) = 4 x 3 + 6 x + 4 a n d f ( x ) = 12 x 2 + 1 f' (x)=4x^3+6x+4 ~and ~f''(x)=12x^2+1 lim x 1 ( f ( x ) . f ( x ) ) = f ( 1 ) f ( 1 ) = ( 14 ) ( 18 ) = 252 \lim _{x\rightarrow 1}\left ( f^{'}(x) . f^{''}(x)\right)=f'(1)f''(1)=(14)(18)=\Large 252

how did u figure out the pattern .. just practice or any trick

Dhruv Aggarwal - 5 years, 1 month ago

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Pure inspection... Otherwise you have to solve nasty equations....

Rishabh Jain - 5 years, 1 month ago

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agreed .. nice solution

Dhruv Aggarwal - 5 years, 1 month ago

The polynomial I got was x^4+5/3 x^3 - 7x^2 + 67/3x - 7, which should definitely work out ok...

Bryan Hung - 5 years, 4 months ago

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