Insufficient 5th degree!

Algebra Level 3

If α \alpha is a real root of the equation x 5 x 3 + x 2 = 0 x^{5}-x^{3}+x-2=0 , Find the value of α 6 \left \lfloor \alpha ^{6} \right \rfloor .


The answer is 3.

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1 solution

Sabhrant Sachan
Dec 24, 2016

i am calculating the answer using approximation. Let \text{i am calculating the answer using approximation. Let }

f ( x ) = x 5 x 3 + x 2 f ( x ) = 5 x 4 3 x 2 + 1 \begin{aligned} f(x) & = x^5 -x^3+x-2 \\ f^{'}(x) & = 5x^4-3x^2+1 \end{aligned}

f ( x ) = lim Δ x 0 f ( x + Δ x ) f ( x ) Δ x f ( x + Δ x ) f ( x ) + Δ x f ( x ) \boxed {f^{'}(x) = \displaystyle \lim_{\Delta x \to 0} \dfrac{f(x+\Delta x) - f(x)}{\Delta x } \\ f(x+\Delta x) \approx f(x) + \Delta x f^{'}(x) }

above f ( 2 ) function increases rapidly below f ( 1 ) function decreases rapidly \text{above } f(2) \text{function increases rapidly } \\ \text{below } f(-1) \text{function decreases rapidly }

f ( 2 ) = 32 8 = 24 a root between 1 and 2 f ( 1 ) = 1 We can safely conclude that α = 1 f ( 0 ) = 2 f ( 1 ) = 3 \begin{aligned} f(2) & = 32-8 =24 \quad \small\color{#3D99F6}{\text{a root between 1 and 2}} \\ f(1) & = -1 \quad \small\color{#3D99F6}{\text{We can safely conclude that }} \lfloor \alpha \rfloor = 1 \\ f(0) & = -2 \\ f(-1) & = -3 \\ \end{aligned}

f ( 1 + Δ x ) = f ( 1 ) + Δ x f ( x ) = 0 Δ x = f ( 1 ) f ( 1 ) = 1 3 f ( 1 + Δ x + Δ x 2 ) = f ( 1 + Δ x ) + Δ x 2 × f ( 1 + Δ x ) = 0 Repeating the process 2nd time gives better accuracy Δ x 2 = f ( 1 + Δ x ) f ( 1 + Δ x ) = f ( 4 3 ) f ( 4 3 ) = 0.1026 \begin{aligned} f(1+\Delta x) & = f(1)+\Delta x f^{'}(x)= 0 \\ \Delta x & = -\dfrac{f(1)}{f^{'}(1)} = \dfrac{1}{3} \\ f(1+\Delta x+\Delta x_2 ) & = f(1+\Delta x )+\Delta x_2\times f^{'}(1+\Delta x)= 0 \quad \small\color{#3D99F6}{\text{Repeating the process 2nd time gives better accuracy }} \\ \Delta x_2 & = -\dfrac{f(1+\Delta x)}{f^{'}(1+\Delta x)} = -\dfrac{f\left(\frac43\right)}{f^{'}\left(\frac43\right)} = -0.1026 \end{aligned}

α = 1 + Δ x + Δ x 2 = 1 + 1 3 0.1026 = 1.2307 α 6 = 3.031 α 6 = 3 \begin{aligned} \alpha & = 1+\Delta x+\Delta x_2 \\ & = 1+\dfrac{1}{3}-0.1026 = 1.2307 \\ \alpha^{6} & = 3.031 \\ \lfloor \alpha^6 \rfloor & = 3 \end{aligned}

Can you think of an algebric method.

Aaron Jerry Ninan - 4 years, 5 months ago

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