Unusual Partition

Algebra Level 4

3 x 1 2 x = 2 \large |3x-|1-2x||=2

Given that the sum of all possible values of x x that satisfy the equation above can be expressed in the form m n \dfrac{m}{n} , where m m and n n are coprime, positive integers, find the value of m + n m+n .

This problem is part of the set Absolute Value Equations .


The answer is 9.

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4 solutions

Hem Shailabh Sahu
Mar 29, 2015

3 x 1 2 x = 2 |3x-|1-2x||=2 has two solutions, viz.

  1. 3 x 1 2 x = 2 3x-|1-2x|=2 3 x 2 = 1 2 x . . . ( A ) \Rightarrow 3x-2=|1-2x|...(A)

  2. 3 x 1 2 x = 2 3x-|1-2x|=-2 3 x + 2 = 1 2 x . . . ( B ) \Rightarrow 3x+2=|1-2x|...(B) .

Further more, each equation - ( A ) (A) & ( B ) (B) will provide us with 2 solutions each for x x . Let's name them x 1 x_1 , x 2 x_2 , x 3 x_3 & x 4 x_4 .

(A) x 1 = 3 5 , x 2 = 1 \Rightarrow x_1=\frac{3}{5}, x_2=1

(B) x 3 = 1 5 , x 4 = 3 \Rightarrow x_3=\frac{-1}{5}, x_4=-3

x 1 x_1 & x 4 x_4 are rejected as they do not satisfy the main equation : 3 x 1 2 x = 2 |3x-|1-2x||=2

Therefore, the values of x x satisfying the given equation are x 2 x_2 & x 3 x_3 , which are 1 , 1 5 1, \frac{-1}{5} respectively.

x 2 + x 3 = 4 5 = m n \Rightarrow x_2+x_3=\frac{4}{5}=\frac{m}{n}

Therefore, m + n = 4 + 5 = 9 m+n =4+5=\boxed{9} .

Let f ( x ) = 3 x 1 2 x f(x) = |3x-|1-2x|| .

Case 1: For x < 1 2 x < \dfrac 12

f ( x ) = 3 x 1 + 2 x = 5 x 1 { For x < 1 5 f ( x ) = 1 5 x = 2 x = 1 5 < 1 5 Accepted For 1 5 x < 1 2 f ( x ) = 5 x 1 0 f ( x ) < 3 2 < 2 No solution f(x) = |3x-1+2x| = |5x-1| \implies \begin{cases}\small \text{For } x < \dfrac 15 & \implies f(x) = 1-5x = 2 & \implies x = - \dfrac 15 < \dfrac 15 & \small \color{#3D99F6} \text{Accepted} \\ \small \text{For } \dfrac 15 \le x < \dfrac 12 & \implies f(x) = 5x - 1 & \implies 0 \le f(x) < \dfrac 32 < 2 & \small \color{#D61F06} \text{No solution} \end{cases}

Case 2: For x 1 2 x \ge \dfrac 12

f ( x ) = 3 x 2 x + 1 = x + 1 = x + 1 = 2 x = 1 1 2 Accepted f(x) = |3x-2x+1| = |x+1| = x+1 = 2 \implies x = 1 \ge \dfrac 12 \quad \small \color{#3D99F6} \text{Accepted}

Therefore, the sum of solution is 1 1 5 = 4 5 1-\dfrac 15 = \frac 45 m + n = 4 + 5 = 9 \implies m+n = 4+5 = \boxed 9 .

Prakhar Gupta
Nov 10, 2014

I will here make 2 cases

i) x 1 2 x\geq\dfrac{1}{2} .

ii) x < 1 2 x<\dfrac{1}{2} .

Solving case 1 we get 3 x + 1 2 x = 2 |3x+1-2x|=2 x + 1 = 2 , x + 1 = 2 x+1=2,x+1=-2 x = 1 , x = 3 x=1,x=-3 Here we will reject x = 3 x=-3 as a solution since it is not in the chosen domain and keep x = 1 x=1 .

Now solving case 2 we get 3 x 1 + 2 x = 2 |3x-1+2x|=2 5 x 1 = 2 , 5 x 1 = 2 5x-1=2,5x-1=-2 x = 3 5 , x = 1 5 x=\dfrac{3}{5} , x={-1}{5} Here we will reject x = 3 5 x=\dfrac{3}{5} and keep x = 1 5 x=\dfrac{-1}{5} .

So we get the sum of 1 1 5 = 4 5 \boxed{1-\dfrac{1}{5}} = \dfrac{4}{5} .

Typo in the third line of case 2 solving. You forgot to put the " \textrm{\frac} " before starting the braces for the second solution.

Prasun Biswas - 6 years, 3 months ago
Tan Yong Boon
Oct 25, 2014

Assume x has a positive value, the equation would then solve for us to obtain x=1.

Assume x has a negative value, the equation would then solve for us to obtain x= -0.2.

Then, add both possible values together and we would obtain 4/5.

Hence, the answer is 4+5=9.

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