A calculus problem by Shashwat Avasthi

Calculus Level 3

If f ( x ) = x 3 + x 2 f ( 1 ) + x f ( 2 ) + f ( 3 ) f(x) = x^3 + x^2 f'(1) + x f''(2) + f'''(3) for all real x x , then find f ( x ) f(x) independent of f ( 1 ) , f ( 2 ) f'(1), f''(2) and f ( 3 ) f'''(3) . Submit your answer as f ( 5 ) f(5) .


The answer is 16.

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2 solutions

f ( x ) = x 3 + x 2 f ( 1 ) + x f ( 2 ) + f ( 3 ) f ( 3 ) = 6 ; f ( 2 ) = 12 + 2 f ( 1 ) ; f(x) = x^3 + x^2 f '(1) + xf ''(2) + f'''(3) \Rightarrow f'''(3) = 6 ; f ''(2) = 12 + 2 f '(1) ; f ( 1 ) = 3 + 2 f ( 1 ) + f ( 2 ) f ( 2 ) = f ( 1 ) 3 = 12 + 2 f ( 1 ) f ' (1) = 3 + 2f '(1) + f ''(2) \Rightarrow f ''(2) = - f'(1) - 3 = 12 + 2f '(1) \Rightarrow f ( 1 ) = 5 f ( 2 ) = 2 f ( 5 ) = 125 125 + 10 + 6 = 16 f '(1) = - 5 \Rightarrow f ''(2) = 2 \Rightarrow f(5) = 125 - 125 + 10 + 6 = 16

Rishabh Jain
Feb 3, 2016

Nice problem... :-]

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