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Question Number 10118 by Gaurav3651 last updated on 25/Jan/17

Let f:R→R be a function such that  f(x)=x^3 +x^2 f′(1)+xf′′(2)+f′′′(3)  for x∈R.  1)What is f(1) equal to?  2)What is f′(1) equal to?  3)What is f′′′(10) equal to?  For this question consider the following:  1) f(2)=f(1)−f(0)  2)f′′(2)−2f′(1)=12  which is/are correct?

$${Let}\:{f}:\mathbb{R}\rightarrow\mathbb{R}\:{be}\:{a}\:{function}\:{such}\:{that} \\ $$$${f}\left({x}\right)={x}^{\mathrm{3}} +{x}^{\mathrm{2}} {f}'\left(\mathrm{1}\right)+{xf}''\left(\mathrm{2}\right)+{f}'''\left(\mathrm{3}\right) \\ $$$${for}\:{x}\in\mathbb{R}. \\ $$$$\left.\mathrm{1}\right){What}\:{is}\:{f}\left(\mathrm{1}\right)\:{equal}\:{to}? \\ $$$$\left.\mathrm{2}\right){What}\:{is}\:{f}'\left(\mathrm{1}\right)\:{equal}\:{to}? \\ $$$$\left.\mathrm{3}\right){What}\:{is}\:{f}'''\left(\mathrm{10}\right)\:{equal}\:{to}? \\ $$$${For}\:{this}\:{question}\:{consider}\:{the}\:{following}: \\ $$$$\left.\mathrm{1}\right)\:{f}\left(\mathrm{2}\right)={f}\left(\mathrm{1}\right)−{f}\left(\mathrm{0}\right) \\ $$$$\left.\mathrm{2}\right){f}''\left(\mathrm{2}\right)−\mathrm{2}{f}'\left(\mathrm{1}\right)=\mathrm{12} \\ $$$${which}\:{is}/{are}\:{correct}? \\ $$

Commented by nume1114 last updated on 25/Jan/17

Answer  1)f(1)=4  2)f′(1)=−5  3)f′′′(10)=6    1) f(2)=f(1)−f(0)  2)f′′(2)−2f′(1)=12  Both of these are correct.

$${Answer} \\ $$$$\left.\mathrm{1}\right){f}\left(\mathrm{1}\right)=\mathrm{4} \\ $$$$\left.\mathrm{2}\right){f}'\left(\mathrm{1}\right)=−\mathrm{5} \\ $$$$\left.\mathrm{3}\right){f}'''\left(\mathrm{10}\right)=\mathrm{6} \\ $$$$ \\ $$$$\left.\mathrm{1}\right)\:{f}\left(\mathrm{2}\right)={f}\left(\mathrm{1}\right)−{f}\left(\mathrm{0}\right) \\ $$$$\left.\mathrm{2}\right){f}''\left(\mathrm{2}\right)−\mathrm{2}{f}'\left(\mathrm{1}\right)=\mathrm{12} \\ $$$$\mathrm{Both}\:\mathrm{of}\:\mathrm{these}\:\mathrm{are}\:\mathrm{correct}. \\ $$$$ \\ $$

Answered by nume1114 last updated on 25/Jan/17

f(x)=x^3 +x^2 f′(1)+xf′′(2)+f′′′(3) ...(A)  f′(x)=3x^2 +2xf′(1)+f′′(2) ...(B)  f′′(x)=6x+2f′(1) ...(C)  f′′′(x)=6 ...(D)  (D)⇒f′′′(3)=6 ...(E)          ⇒f′′′(10)=6  (B)⇒f′(1)=3+2f′(1)+f′′(2)          ⇒f′(1)+f′′(2)=−3 ...(F)  (C)⇒f′′(2)=12+2f′(1) ...(G)          ⇒f′′(2)−2f′(1)=^! 12  (F),(G)⇒f′(1)+[12+2f′(1)]=−3                   ⇒f′(1)=−5 ...(H)                   ⇒f′′(2)=2 ...(I)  (A),(E),(H),(I)  ⇒f(x)=x^3 −5x^2 +2x+6  ⇒f(1)=4  ⇒f(2)=−2  ⇒f(0)=6  ⇒f(2)=^! f(1)−f(0)

$${f}\left({x}\right)={x}^{\mathrm{3}} +{x}^{\mathrm{2}} {f}'\left(\mathrm{1}\right)+{xf}''\left(\mathrm{2}\right)+{f}'''\left(\mathrm{3}\right)\:...\left({A}\right) \\ $$$${f}'\left({x}\right)=\mathrm{3}{x}^{\mathrm{2}} +\mathrm{2}{xf}'\left(\mathrm{1}\right)+{f}''\left(\mathrm{2}\right)\:...\left({B}\right) \\ $$$${f}''\left({x}\right)=\mathrm{6}{x}+\mathrm{2}{f}'\left(\mathrm{1}\right)\:...\left({C}\right) \\ $$$${f}'''\left({x}\right)=\mathrm{6}\:...\left({D}\right) \\ $$$$\left({D}\right)\Rightarrow{f}'''\left(\mathrm{3}\right)=\mathrm{6}\:...\left({E}\right) \\ $$$$\:\:\:\:\:\:\:\:\Rightarrow{f}'''\left(\mathrm{10}\right)=\mathrm{6} \\ $$$$\left({B}\right)\Rightarrow{f}'\left(\mathrm{1}\right)=\mathrm{3}+\mathrm{2}{f}'\left(\mathrm{1}\right)+{f}''\left(\mathrm{2}\right) \\ $$$$\:\:\:\:\:\:\:\:\Rightarrow{f}'\left(\mathrm{1}\right)+{f}''\left(\mathrm{2}\right)=−\mathrm{3}\:...\left({F}\right) \\ $$$$\left({C}\right)\Rightarrow{f}''\left(\mathrm{2}\right)=\mathrm{12}+\mathrm{2}{f}'\left(\mathrm{1}\right)\:...\left({G}\right) \\ $$$$\:\:\:\:\:\:\:\:\Rightarrow{f}''\left(\mathrm{2}\right)−\mathrm{2}{f}'\left(\mathrm{1}\right)\overset{!} {=}\mathrm{12} \\ $$$$\left({F}\right),\left({G}\right)\Rightarrow{f}'\left(\mathrm{1}\right)+\left[\mathrm{12}+\mathrm{2}{f}'\left(\mathrm{1}\right)\right]=−\mathrm{3} \\ $$$$\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\Rightarrow{f}'\left(\mathrm{1}\right)=−\mathrm{5}\:...\left({H}\right) \\ $$$$\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\Rightarrow{f}''\left(\mathrm{2}\right)=\mathrm{2}\:...\left({I}\right) \\ $$$$\left({A}\right),\left({E}\right),\left({H}\right),\left({I}\right) \\ $$$$\Rightarrow{f}\left({x}\right)={x}^{\mathrm{3}} −\mathrm{5}{x}^{\mathrm{2}} +\mathrm{2}{x}+\mathrm{6} \\ $$$$\Rightarrow{f}\left(\mathrm{1}\right)=\mathrm{4} \\ $$$$\Rightarrow{f}\left(\mathrm{2}\right)=−\mathrm{2} \\ $$$$\Rightarrow{f}\left(\mathrm{0}\right)=\mathrm{6} \\ $$$$\Rightarrow{f}\left(\mathrm{2}\right)\overset{!} {=}{f}\left(\mathrm{1}\right)−{f}\left(\mathrm{0}\right) \\ $$

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