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Algebra Difficulty 2.9 Junior Find the answer

For any positive real number x,xx, \lfloor x \rfloor denotes the largest integer less than or equal to xx. If xx=36\lfloor x \rfloor \cdot x = 36 and yy=71\lfloor y \rfloor \cdot y = 71 where x,y>0x, y > 0, what is x+yx + y equal to?

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

For any positive real number x,xx, \lfloor x \rfloor equals the largest integer less than or equal to xx and so xx\lfloor x \rfloor \leq x. In particular, xxxx=x2\lfloor x \rfloor \cdot x \leq x \cdot x = x^{2}. Thus, if xx=36\lfloor x \rfloor \cdot x = 36, then 36x236 \leq x^{2}. Since x>0x > 0, then x6x \geq 6. In fact, if x=6x = 6, then x=6=6\lfloor x \rfloor = \lfloor 6 \rfloor = 6 and so xx=x2=36\lfloor x \rfloor \cdot x = x^{2} = 36. Therefore, x=6x = 6. (Note that if x>6x > 6, then xx>66=36\lfloor x \rfloor \cdot x > 6 \cdot 6 = 36.) Also, since yy=71\lfloor y \rfloor \cdot y = 71, then y271y^{2} \geq 71. Since y>0y > 0, then y718.43y \geq \sqrt{71} \approx 8.43. Since y718.43y \geq \sqrt{71} \approx 8.43, then y8\lfloor y \rfloor \geq 8. Suppose that y=8\lfloor y \rfloor = 8. In this case, y=71y=718=8.875y = \frac{71}{\lfloor y \rfloor} = \frac{71}{8} = 8.875. Note that if y=718y = \frac{71}{8}, then y=8\lfloor y \rfloor = 8, so y=718y = \frac{71}{8} is a solution. Therefore, x+y=6+718=1198x + y = 6 + \frac{71}{8} = \frac{119}{8}.

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