Maths Olympiad Prep

Track / Stage 6 / 390 of 400 #1390 of 1964

Problem 1390

National olympiad, first round
Algebra Difficulty 6.9 Find the answer

Find all real values of x>1 x>1 which satisfy:

x2x\minus1\plusx\minus1\plusx\minus1x2\equalx\minus1x2\plus1x\minus1\plusx2x\minus1 \frac{x^2}{x\minus{}1} \plus{} \sqrt{x\minus{}1} \plus{}\frac{\sqrt{x\minus{}1}}{x^2} \equal{} \frac{x\minus{}1}{x^2} \plus{} \frac{1}{\sqrt{x\minus{}1}} \plus{} \frac{x^2}{\sqrt{x\minus{}1}}

A number or a short expression. Spacing, $ signs and \frac vs / are all fine.

Official solution

1. Let a=x2x1 a = \frac{x^2}{x-1} , b=x1 b = \sqrt{x-1} , and c=x1x2 c = \frac{\sqrt{x-1}}{x^2} . Notice that abc=1 abc = 1 because:
abc=(x2x1)(x1)(x1x2)=x2x1x1(x1)x2=x2(x1)(x1)x2=1 abc = \left( \frac{x^2}{x-1} \right) \left( \sqrt{x-1} \right) \left( \frac{\sqrt{x-1}}{x^2} \right) = \frac{x^2 \cdot \sqrt{x-1} \cdot \sqrt{x-1}}{(x-1) \cdot x^2} = \frac{x^2 \cdot (x-1)}{(x-1) \cdot x^2} = 1

2. Given the equation:
a+b+c=1a+1b+1c a + b + c = \frac{1}{a} + \frac{1}{b} + \frac{1}{c}
Substitute a a , b b , and c c :
x2x1+x1+x1x2=x1x2+1x1+x2x1 \frac{x^2}{x-1} + \sqrt{x-1} + \frac{\sqrt{x-1}}{x^2} = \frac{x-1}{x^2} + \frac{1}{\sqrt{x-1}} + \frac{x^2}{\sqrt{x-1}}

3. Using the fact that abc=1 abc = 1 , we can rewrite the equation as:
a+b+c=1a+1b+1c a + b + c = \frac{1}{a} + \frac{1}{b} + \frac{1}{c}
This implies:
a+b+c=1a+1b+1c a + b + c = \frac{1}{a} + \frac{1}{b} + \frac{1}{c}

4. We can rewrite the equation as:
a+b+c=1a+1b+1c a + b + c = \frac{1}{a} + \frac{1}{b} + \frac{1}{c}
Since abc=1 abc = 1 , we have:
a+b+1ab=1a+1b+ab a + b + \frac{1}{ab} = \frac{1}{a} + \frac{1}{b} + ab

5. Rearrange the terms:
a+b+1ab=1a+1b+ab a + b + \frac{1}{ab} = \frac{1}{a} + \frac{1}{b} + ab
This can be factored as:
(a1)(b1)(ab1)=0 (a-1)(b-1)(ab-1) = 0

6. Solving each factor:
- If a=1 a = 1 :
x2x1=1    x2=x1    x2x+1=0 \frac{x^2}{x-1} = 1 \implies x^2 = x-1 \implies x^2 - x + 1 = 0
The discriminant of this quadratic equation is:
Δ=(1)2411=14=3 \Delta = (-1)^2 - 4 \cdot 1 \cdot 1 = 1 - 4 = -3
Since the discriminant is negative, there are no real solutions for x x .

- If b=1 b = 1 :
x1=1    x1=1    x=2 \sqrt{x-1} = 1 \implies x-1 = 1 \implies x = 2

- If ab=1 ab = 1 :
x2x1x1=1    x2x1x1=1    x2x1=x1 \frac{x^2}{x-1} \cdot \sqrt{x-1} = 1 \implies \frac{x^2 \sqrt{x-1}}{x-1} = 1 \implies x^2 \sqrt{x-1} = x-1
Let y=x1 y = \sqrt{x-1} , then y2=x1 y^2 = x-1 and the equation becomes:
x2y=y2    x2=y    x2=x1 x^2 y = y^2 \implies x^2 = y \implies x^2 = \sqrt{x-1}
Squaring both sides:
x4=x1    x4x+1=0 x^4 = x-1 \implies x^4 - x + 1 = 0
This is a quartic equation and solving it analytically is complex. However, we already have x=2 x = 2 as a solution from the previous factor.

7. Verify x=2 x = 2 :
a=2221=4,b=21=1,c=2122=14 a = \frac{2^2}{2-1} = 4, \quad b = \sqrt{2-1} = 1, \quad c = \frac{\sqrt{2-1}}{2^2} = \frac{1}{4}
a+b+c=4+1+14=5.25 a + b + c = 4 + 1 + \frac{1}{4} = 5.25
1a+1b+1c=14+1+4=5.25 \frac{1}{a} + \frac{1}{b} + \frac{1}{c} = \frac{1}{4} + 1 + 4 = 5.25
Both sides are equal, confirming x=2 x = 2 is a solution.

The final answer is x=2 \boxed{ x = 2 }

Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic, difficulty and ordering added by this site.