An algebra problem by Wildan Bagus Wicaksono

Algebra Level 3

x = 5 3 ( 16 25 3 4 25 3 + 1 25 3 ) 1 x=\sqrt [ 3 ]{ 5 } { \left( \sqrt [ 3 ]{ \frac { 16 }{ 25 } } -\sqrt [ 3 ]{ \frac { 4 }{ 25 } } +\sqrt [ 3 ]{ \frac { 1 }{ 25 } } \right) }^{ -1 }

For x x as defined above, determine the value of x \left\lceil x \right\rceil .

Notation: \lceil \cdot \rceil denotes the ceiling function.


The answer is 3.

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2 solutions

x = 5 3 ( 16 25 3 4 25 3 + 1 25 3 ) 1 = 5 3 16 25 3 4 25 3 + 1 25 3 = 5 3 25 3 16 3 4 3 + 1 = 5 ( 4 3 + 1 ) ( 4 3 + 1 ) ( ( 4 3 ) 2 4 3 + 1 ) = 5 ( 4 3 + 1 ) ( 4 3 ) 3 + 1 = 5 ( 4 3 + 1 ) 5 = 4 3 + 1 \begin{aligned} x &= \sqrt[3]5 \left(\sqrt[3]{\frac{16}{25}} - \sqrt[3]{\frac{4}{25}} + \sqrt[3]{\frac{1}{25}} \right)^{-1} \\ &= \frac {\sqrt[3]5}{\sqrt[3]{\frac{16}{25}} - \sqrt[3]{\frac{4}{25}} + \sqrt[3]{\frac{1}{25}}} \\ &= \frac {\sqrt[3]5 \sqrt[3]{25}}{\sqrt[3]{16} - \sqrt[3]4 + 1} \\ &= \frac {5(\sqrt[3]4 + 1)}{(\sqrt[3]4 + 1)((\sqrt[3]4)^2 - \sqrt[3]4 + 1)} \\ &= \frac {5(\sqrt[3]4 + 1)}{(\sqrt[3]4)^3 + 1} \\ &= \frac {5(\sqrt[3]4 + 1)}5 \\ &= \sqrt[3]4 + 1 \end{aligned}

x = 4 3 + 1 = 3 \implies \lceil x \rceil = \lceil \sqrt[3]4 + 1 \rceil = \boxed{3}

= 5 3 ( 16 25 3 4 25 3 + 1 25 3 ) 1 = 5 3 ( 1 25 3 { 16 3 4 3 + 1 } ) 1 =\sqrt [ 3 ]{ 5 } { \left( \sqrt [ 3 ]{ \frac { 16 }{ 25 } } -\sqrt [ 3 ]{ \frac { 4 }{ 25 } } +\sqrt [ 3 ]{ \frac { 1 }{ 25 } } \right) }^{ -1 }\\ =\sqrt [ 3 ]{ 5 } { \left( \frac { 1 }{ \sqrt [ 3 ]{ 25 } } \left\{ \sqrt [ 3 ]{ 16 } -\sqrt [ 3 ]{ 4 } +1 \right\} \right) }^{ -1 }

Let y = 16 3 4 3 + 1 y=\sqrt [ 3 ]{ 16 } -\sqrt [ 3 ]{ 4 } +1 .

y = 16 3 4 3 + 1 4 3 y = 4 16 3 + 4 3 + ( 1 + 4 3 ) y = 5 y = 5 1 + 4 3 y=\sqrt [ 3 ]{ 16 } -\sqrt [ 3 ]{ 4 } +1\quad \\ \sqrt [ 3 ]{ 4 } y=4-\sqrt [ 3 ]{ 16 } +\sqrt [ 3 ]{ 4 } \\ -----------------+\\ \left( 1+\sqrt [ 3 ]{ 4 } \right) y=5\\ y=\frac { 5 }{ 1+\sqrt [ 3 ]{ 4 } }

x = 5 3 ( 1 25 3 5 1 + 4 3 ) 1 x = 5 3 25 3 ( 1 + 4 3 ) 5 x = 1 + 4 3 x = 1 + 1 , . . . x = 2 , . . . x = 3 x=\sqrt [ 3 ]{ 5 } { \left( \frac { 1 }{ \sqrt [ 3 ]{ 25 } } \cdot \frac { 5 }{ 1+\sqrt [ 3 ]{ 4 } } \right) }^{ -1 }\\ x=\frac { \sqrt [ 3 ]{ 5 } \cdot \sqrt [ 3 ]{ 25 } \left( 1+\sqrt [ 3 ]{ 4 } \right) }{ 5 } \\ x=1+\sqrt [ 3 ]{ 4 } \\ x=1+1,...\\ x=2,...\\ \left\lceil x \right\rceil =3

So, x = 3 \left\lceil x \right\rceil =\boxed { 3 } .

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