Solution set!

Algebra Level 4

2 x + 1 3 x 2 > 2 \large \frac{2x+1}{3x-2} > 2

If the solution set of the inequality above is given by ( a , b ) (a,b) .

And if the value of a b = m n ab = \dfrac{m}{n} , where m m and n n are coprime positive integers, find the value of m + n m+n .


The answer is 11.

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3 solutions

Chew-Seong Cheong
Jan 13, 2017

2 x + 1 3 x 2 > 2 \begin{aligned} \frac {2x+1}{3x-2} & > 2 \end{aligned}

{ 2 x + 1 > 2 ( 3 x 2 ) for 3 x 2 > 0 x > 2 3 . . . ( 1 ) 2 x + 1 < 2 ( 3 x 2 ) for 3 x 2 < 0 x < 2 3 . . . ( 2 ) \implies \begin{cases} 2x+1 \ {\color{#3D99F6}>} \ 2(3x-2) & \text{for }3x - 2 \ {\color{#3D99F6}>} \ 0 & \implies x \ {\color{#3D99F6}>} \ \dfrac 23 & ...(1) \\ 2x+1 \ {\color{#D61F06}<} \ 2(3x-2) & \text{for }3x - 2 \ {\color{#D61F06}<} \ 0 & \implies x \ {\color{#D61F06}<} \ \dfrac 23 & ... (2) \end{cases}

( 1 ) : 2 x + 1 > 6 x 4 for x > 2 3 4 x < 5 2 3 < x < 5 4 A solution. \begin{aligned} (1): \quad 2x + 1 & > 6x -4 & \text{for } x > \frac 23 \\ 4x & < 5 \\ \implies \frac 23 < x & < \frac 54 & \color{#3D99F6} \text{A solution.} \end{aligned}

( 2 ) : 2 x + 1 < 6 x 4 for x < 2 3 4 x > 5 2 3 > x > 5 4 No solution. \begin{aligned} (2): \quad 2x + 1 & < 6x -4 & \text{for } x < \frac 23 \\ 4x & > 5 \\ \implies \frac 23 > x & > \frac 54 & \color{#D61F06} \text{No solution.} \end{aligned}

Therefore, 2 3 < x < 5 4 \dfrac 23 < x < \dfrac 54 a b = 2 3 × 5 4 = 5 6 \implies ab = \dfrac 23 \times \dfrac 54 = \dfrac 56 m + n = 5 + 6 = 11 \implies m+n = 5+6 = \boxed{11}

When you multiply both sides by 3x-2, don't you have to split into cases depending on the sign of 3x-2? You ended up catching it later, but I don't think your second step was valid.

Nick Schreiner - 4 years, 5 months ago

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Thanks you are right.

Chew-Seong Cheong - 4 years, 5 months ago

2 x + 1 3 x 2 > 2 \frac{2x+1}{3x-2}>2

2 x + 1 3 x 2 2 > 0 \frac{2x+1}{3x-2}-2>0

2 x + 1 6 x + 4 3 x 2 > 0 \frac{2x+1-6x+4}{3x-2}>0

5 4 x 3 x 2 > 0 \frac{5-4x}{3x-2}>0

4 x 5 3 x 2 < 0 \frac{4x-5}{3x-2}<0

So ( a , b ) (a,b) is ( 2 3 , 5 4 ) (\frac{2}{3},\frac{5}{4}) a b = 5 6 ab=\frac{5}{6}

Zee Ell
Jan 13, 2017

2 x + 1 3 x 2 > 2 \frac {2x+1}{3x-2} > 2

Let's evaluate the following two cases:

Case 1: 3 x 2 > 0 x > 2 3 \text {Case 1: } 3x - 2 > 0 \iff x > \frac {2}{3}

2 x + 1 > 2 ( 3 x 2 ) 2x + 1 > 2(3x -2)

2 x + 1 > 6 x 4 2x + 1 > 6x - 4

5 > 4 x 5 > 4x

5 4 > x \frac {5}{4} > x

Hence, our solution in this case:

2 3 < x < 5 4 \frac {2}{3} < x < \frac {5}{4}

Case 2: 3 x 2 < 0 x < 2 3 \text {Case 2: } 3x - 2 < 0 \iff x < \frac {2}{3}

Multiplying by a negative number changes the direction of the inequality to the opposite:

2 x + 1 < 2 ( 3 x 2 ) 2x + 1 < 2(3x -2)

2 x + 1 < 6 x 4 2x + 1 < 6x - 4

5 < 4 x 5 < 4x

5 4 < x \frac {5}{4} < x

Since x < 2 3 < 5 4 , therefore we don’t have a solution in this case. \text {Since } x < \frac {2}{3} < \frac {5}{4} \text { , therefore we don't have a solution in this case.}

This means, that our only solution is:

x ( 2 3 , 5 4 ) x \in ( \frac {2}{3} , \frac {5}{4} )

a = 2 3 a = \frac {2}{3}

b = 5 4 b = \frac {5}{4}

a b = 2 3 × 5 4 = 10 12 = 5 6 ab = \frac {2}{3} × \frac {5}{4} = \frac {10}{12} = \frac {5}{6}

Hence m = 5, n = 6 and our solution should be:

m + n = 5 + 6 = 11 m + n = 5 + 6 = \boxed {11}

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