Simple Simplification III

Algebra Level 4

2 31 + 3 31 2 29 + 3 29 = ? \large \left\lfloor \dfrac{2^{31}+3^{31}}{2^{29}+3^{29}}\right\rfloor = \ ?


The answer is 8.

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3 solutions

2 31 + 3 31 2 29 + 3 29 = 3 31 ( 1 + 2 3 31 ) 3 29 ( 1 + 2 3 29 ) = 9 ( 1 + 2 3 31 ) ( 1 + 2 3 29 ) \quad \quad \left\lfloor \frac { { 2 }^{ 31 }{ +3 }^{ 31 } }{ { 2 }^{ 29 }+{ 3 }^{ 29 } } \right\rfloor \\ \\ =\quad \left\lfloor \frac { { 3 }^{ 31 }(1+{ \frac { 2 }{ 3 } }^{ 31 }) }{ { 3 }^{ 29 }(1+{ \frac { 2 }{ 3 } }^{ 29 }) } \right\rfloor \\ \\ =\quad \left\lfloor \frac { 9(1+{ \frac { 2 }{ 3 } }^{ 31 }) }{ (1+{ \frac { 2 }{ 3 } }^{ 29 }) } \right\rfloor

Now in the floor function, we can see that 2 / 3 2/3 is less than 1. So, 2 / 3 2/3 raised to the power 29 29 and 31 31 is also less than 1. But,

( 2 3 ) 29 > ( 2 3 ) 31 1 + ( 2 3 ) 29 > 1 + ( 2 3 ) 31 1 + ( 2 3 ) 31 1 + ( 2 3 ) 29 < 1 9 ( 1 + ( 2 3 ) 31 1 + ( 2 3 ) 29 ) < 9 9 ( 1 + ( 2 3 ) 31 1 + ( 2 3 ) 29 ) = 8 \quad \quad \quad { (\frac { 2 }{ 3 } })^{ 29 }\quad >{ \quad (\frac { 2 }{ 3 } })^{ 31 }\\ \Rightarrow \quad 1+{ (\frac { 2 }{ 3 } })^{ 29 }\quad >\quad 1+({ \frac { 2 }{ 3 } })^{ 31 }\\ \Rightarrow \quad \frac { 1+{ (\frac { 2 }{ 3 } })^{ 31 } }{ 1+{ (\frac { 2 }{ 3 } })^{ 29 } } \quad <\quad 1\\ \\ \Rightarrow \quad 9(\frac { 1+{ (\frac { 2 }{ 3 } })^{ 31 } }{ 1+{ (\frac { 2 }{ 3 } })^{ 29 } } )\quad <\quad 9\\ \\ \Rightarrow \quad \left\lfloor 9(\frac { 1+{ (\frac { 2 }{ 3 } })^{ 31 } }{ 1+{ (\frac { 2 }{ 3 } })^{ 29 } } ) \right\rfloor \quad =\quad 8

Which is the required value!!!

Danish Ahmed
Mar 15, 2015

2 31 + 2 31 2 29 + 3 29 \dfrac{2^{31}+2^{31}}{2^{29}+3^{29}}

= ( 2 29 + 3 29 ) ( 2 2 + 3 2 ) 4 ( 2 29 + 3 29 ) 5 2 29 2 29 + 3 29 = \dfrac{(2^{29}+3^{29})(2^2+3^2)-4(2^{29}+3^{29})-5 \cdot 2^{29}}{2^{29}+3^{29}}

= 13 4 5 2 29 3 29 + 2 29 = 13-4- \dfrac{5 \cdot 2^{29}}{3^{29}+2^{29}}

It can be easily seen that 5 2 29 3 29 + 2 29 \dfrac{5 \cdot 2^{29}}{3^{29}+2^{29}} is less that 1 1

it follows that 8 < 2 31 + 2 31 2 29 + 3 29 < 9 8 < \dfrac{2^{31}+2^{31}}{2^{29}+3^{29}} < 9

So the answer is 8 \boxed{8}

Correct the first thing that you have written

Vaibhav Prasad - 6 years, 3 months ago
Chew-Seong Cheong
Sep 26, 2015

2 31 + 3 31 2 29 + 3 29 = 4 ( 2 29 ) + 9 ( 3 29 ) 2 29 + 3 29 = 4 + 5 ( 3 29 ) 2 29 + 3 29 = 4 + 3 29 + 4 ( 3 29 ) 2 29 + 3 29 We note that 3 29 = ( 2 + 1 ) 29 = 2 29 + 29 ( 2 28 ) + . . . > 4 ( 2 29 ) = 4 + 3 29 4 ( 2 29 ) + 4 ( 2 29 ) + 4 ( 3 29 ) 2 29 + 3 29 = 4 + 4 + 3 29 4 ( 2 29 ) 2 29 + 3 29 Since 3 29 4 ( 2 29 ) 2 29 + 3 29 < 1 2 31 + 3 31 2 29 + 3 29 = 8 \begin{aligned} \frac{2^{31}+3^{31}}{2^{29}+3^{29}} & = \frac{4(2^{29})+9(3^{29})}{2^{29}+3^{29}} \\ & = 4 + \frac{5(3^{29})}{2^{29}+3^{29}} \\ & = 4 + \frac{\color{#3D99F6}{3^{29}} + 4(3^{29})}{2^{29}+3^{29}} \quad \quad \quad \color{#3D99F6}{\text{We note that }3^{29} = (2+1)^{29} = 2^{29} + 29(2^{28})+... > 4(2^{29})} \\ & = 4 + \frac{\color{#3D99F6}{3^{29} - 4(2^{29}) + 4(2^{29})} + 4(3^{29})}{2^{29}+3^{29}} \\ & = 4 + 4 + \color{#D61F06}{\frac{3^{29} - 4(2^{29})}{2^{29}+3^{29}}} \quad \quad \color{#D61F06}{\text{Since }\frac{3^{29} - 4(2^{29})}{2^{29}+3^{29}}<1} \\ \Rightarrow \left \lfloor \frac{2^{31}+3^{31}}{2^{29}+3^{29}} \right \rfloor & = \boxed{8} \end{aligned}

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