Maths Olympiad Prep

Track / Stage 6 / 306 of 400 #1306 of 1964

Problem 1306

National olympiad, first round
Algebra Difficulty 6.5 Multiple choice

There exists a fraction xx that satisfies x2+5x=13 \sqrt{x^2+5} - x = \tfrac{1}{3}. What is the sum of the numerator and denominator of this fraction?

Pick one

Official solution

1. Start with the given equation:
x2+5x=13 \sqrt{x^2 + 5} - x = \frac{1}{3}

2. Rearrange the equation to isolate the square root term:
x2+5=x+13 \sqrt{x^2 + 5} = x + \frac{1}{3}

3. Square both sides to eliminate the square root:
(x2+5)2=(x+13)2 (\sqrt{x^2 + 5})^2 = \left(x + \frac{1}{3}\right)^2
x2+5=x2+23x+19 x^2 + 5 = x^2 + \frac{2}{3}x + \frac{1}{9}

4. Simplify the equation by subtracting x2x^2 from both sides:
5=23x+19 5 = \frac{2}{3}x + \frac{1}{9}

5. Clear the fraction by multiplying every term by 9:
45=6x+1 45 = 6x + 1

6. Isolate xx by subtracting 1 from both sides:
44=6x 44 = 6x

7. Solve for xx by dividing both sides by 6:
x=446=223 x = \frac{44}{6} = \frac{22}{3}

8. Verify that x=223x = \frac{22}{3} satisfies the original equation:
(223)2+5223=13 \sqrt{\left(\frac{22}{3}\right)^2 + 5} - \frac{22}{3} = \frac{1}{3}
4849+5223=13 \sqrt{\frac{484}{9} + 5} - \frac{22}{3} = \frac{1}{3}
4849+459223=13 \sqrt{\frac{484}{9} + \frac{45}{9}} - \frac{22}{3} = \frac{1}{3}
5299223=13 \sqrt{\frac{529}{9}} - \frac{22}{3} = \frac{1}{3}
233223=13 \frac{23}{3} - \frac{22}{3} = \frac{1}{3}
13=13 \frac{1}{3} = \frac{1}{3}

The solution is verified.

9. The fraction x=223x = \frac{22}{3} has a numerator of 22 and a denominator of 3. The sum of the numerator and denominator is:
22+3=25 22 + 3 = 25

The final answer is 25\boxed{25}.

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