Have You Tried Adding Floors?

Algebra Level 5

2 x + 1 3 + 4 x + 5 6 = 3 x 1 2 \left\lfloor \dfrac { 2x+1 }{ 3 } \right\rfloor +\left\lfloor \dfrac { 4x+5 }{ 6 } \right\rfloor =\dfrac { 3x-1 }{ 2 } Find the number of real values of x x satisfying the above equation.


The answer is 9.

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1 solution

Since the LHS is an integer, the RHS must also be an integer n n , 3 x 1 2 = n \Rightarrow \dfrac{3x-1}{2} = n and x = 2 n + 1 3 x = \dfrac{2n+1}{3} . And that:

2 x + 1 3 + 4 x + 5 6 = 3 x 1 2 2 ( 2 n + 1 3 ) + 1 3 + 4 ( 2 n + 1 3 ) + 5 6 = n 4 n + 5 9 + 8 n + 19 18 = n 4 n + 5 9 + 8 n + 1 18 + 1 = n \begin{aligned} \left \lfloor \frac{2x+1}{3} \right \rfloor + \left \lfloor \frac{4x+5}{6} \right \rfloor & = \frac{3x-1}{2} \\ \left \lfloor \frac{2\left(\frac{2n+1}{3}\right)+1}{3} \right \rfloor + \left \lfloor \frac{4\left(\frac{2n+1}{3}\right)+5}{6} \right \rfloor & = n \\ \left \lfloor \frac{4n+5}{9} \right \rfloor + \left \lfloor \frac{8n+19}{18} \right \rfloor & = n \\ \color{#3D99F6}{\left \lfloor \frac{4n+5}{9} \right \rfloor} + \color{#D61F06}{\left \lfloor \frac{8n+1}{18} \right \rfloor} + 1 & = n \end{aligned}

We note that the LHS increases with n n and LHS > > RHS for n 1 n \le 1 and LHS < < RHS for n 11 n \ge 11 . And when:

{ n = 2 , 1 + 0 + 1 = 2 , acceptable n = 3 , 1 + 1 + 1 = 3 , acceptable n = 4 , 2 + 1 + 1 = 4 , acceptable n = 5 , 2 + 2 + 1 = 5 , acceptable n = 6 , 3 + 2 + 1 = 6 , acceptable n = 7 , 3 + 3 + 1 = 7 , acceptable n = 8 , 4 + 3 + 1 = 8 , acceptable n = 9 , 4 + 4 + 1 = 9 , acceptable n = 10 , 5 + 4 + 1 = 10 , acceptable \begin{cases} n = 2, & \Rightarrow \color{#3D99F6}{1} + \color{#D61F06}{0} + 1 = 2, & \text{acceptable} \\ n = 3, & \Rightarrow \color{#3D99F6}{1} + \color{#D61F06}{1} + 1 = 3, & \text{acceptable} \\ n = 4, & \Rightarrow \color{#3D99F6}{2} + \color{#D61F06}{1} + 1 = 4, & \text{acceptable} \\ n = 5, & \Rightarrow \color{#3D99F6}{2} + \color{#D61F06}{2} + 1 = 5, & \text{acceptable} \\ n = 6, & \Rightarrow \color{#3D99F6}{3} + \color{#D61F06}{2} + 1 = 6, & \text{acceptable} \\ n = 7, & \Rightarrow \color{#3D99F6}{3} + \color{#D61F06}{3} + 1 = 7, & \text{acceptable} \\ n = 8, & \Rightarrow \color{#3D99F6}{4} + \color{#D61F06}{3} + 1 = 8, & \text{acceptable} \\ n = 9, & \Rightarrow \color{#3D99F6}{4} + \color{#D61F06}{4} + 1 = 9, & \text{acceptable} \\ n = 10, & \Rightarrow \color{#3D99F6}{5} + \color{#D61F06}{4} + 1 = 10, & \text{acceptable} \\ \end{cases}

There are 9 \boxed{9} real values of x x satisfying the equation.

Is there a formal proof for that L H S < R H S LHS<RHS for n > 10 n>10 ?

Arihant Samar - 5 years, 3 months ago

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No, but it is obvious that LHS decreases faster than RHS.

Chew-Seong Cheong - 5 years, 3 months ago

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We know that the LHS increase with n n although it is smaller than its continuous form as follows.

4 n + 5 9 + 8 n + 19 18 4 n + 5 9 + 8 n + 19 18 \begin{aligned} \left \lfloor \frac{4n+5}{9} \right \rfloor + \left \lfloor \frac{8n+19}{18} \right \rfloor & \le \frac{4n+5}{9} + \frac{8n+19}{18} \end{aligned}

Consider the following:

4 n + 5 9 + 8 n + 19 18 n 8 9 n + 29 18 n 1 9 n 29 18 n 14.5 \begin{aligned} \frac{4n+5}{9} + \frac{8n+19}{18} & \le n \\ \frac{8}{9}n + \frac{29}{18} \le n \\ \frac{1}{9}n \ge \frac{29}{18} \\ n \ge 14.5 \end{aligned}

Therefore, the LHS is smaller than RHS at an n n . In this case it is n = 11 n=11 .

Chew-Seong Cheong - 5 years, 3 months ago

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