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Algebra Difficulty 6.2 National Olympiad Prove it Philippines

Problem:
If ff is a function such that f(a+b)=1f(a)+1f(b)f(a+b) = \frac{1}{f(a)} + \frac{1}{f(b)}, find all possible values of f(2011)f(2011).

Solution

Solution:
Consider f(0)=f(0+0)=1f(0)+1f(0)f(0) = f(0+0) = \frac{1}{f(0)} + \frac{1}{f(0)} which gives [f(0)]2=2[f(0)]^2 = 2. Thus, f(0)=±2f(0) = \pm \sqrt{2}.

Let x=f(2011)x = f(2011).

If f(0)=2f(0) = \sqrt{2} then x=f(2011)=f(2011+0)=1f(2011)+1f(0)=1x+12x = f(2011) = f(2011+0) = \frac{1}{f(2011)} + \frac{1}{f(0)} = \frac{1}{x} + \frac{1}{\sqrt{2}}. So, x=2+x2xx = \frac{\sqrt{2} + x}{\sqrt{2} x} which yields 2x2x2=0\sqrt{2} x^2 - x - \sqrt{2} = 0.

Solving for xx using the quadratic formula yields:
x=1±14(2)(2)22=1±322 x = \frac{1 \pm \sqrt{1 - 4(\sqrt{2})(-\sqrt{2})}}{2 \sqrt{2}} = \frac{1 \pm 3}{2 \sqrt{2}}
Hence, if f(0)=2f(0) = \sqrt{2} then either f(2011)=2f(2011) = \sqrt{2} or f(2011)=22f(2011) = -\frac{\sqrt{2}}{2}.

However, suppose f(2011)=22f(2011) = -\frac{\sqrt{2}}{2}. Consider f(0)=f(2011+(2011))=1f(2011)+1f(2011)f(0) = f(2011 + (-2011)) = \frac{1}{f(2011)} + \frac{1}{f(-2011)} which implies that 2=2+1f(2011)\sqrt{2} = -\sqrt{2} + \frac{1}{f(-2011)}. Thus, f(2011)=24f(-2011) = \frac{\sqrt{2}}{4}.

But if we consider f(2011)=f(2011+0)=1f(2011)+1f(0)f(-2011) = f(-2011+0) = \frac{1}{f(-2011)} + \frac{1}{f(0)}, this means that 24=22+1f(0)\frac{\sqrt{2}}{4} = 2\sqrt{2} + \frac{1}{f(0)}.
Thus, f(0)=227f(0) = -\frac{2\sqrt{2}}{7} which is a contradiction. Thus for f(0)=2f(0) = \sqrt{2}, f(2011)=2f(2011) = \sqrt{2}.

If f(0)=2f(0) = -\sqrt{2} then x=f(2011)=f(2011+0)=1f(2011)+1f(0)=1x12x = f(2011) = f(2011+0) = \frac{1}{f(2011)} + \frac{1}{f(0)} = \frac{1}{x} - \frac{1}{\sqrt{2}}. So, x=2x2xx = \frac{\sqrt{2} - x}{\sqrt{2} x} which yields 2x2+x2=0\sqrt{2} x^2 + x - \sqrt{2} = 0.

Solving for xx using the quadratic formula yields:
x=1±14(2)(2)22=1±322 x = \frac{-1 \pm \sqrt{1 - 4(\sqrt{2})(-\sqrt{2})}}{2 \sqrt{2}} = \frac{-1 \pm 3}{2 \sqrt{2}}
Hence, if f(0)=2f(0) = -\sqrt{2} then either f(2011)=2f(2011) = -\sqrt{2} or f(2011)=22f(2011) = \frac{\sqrt{2}}{2}.

However, suppose f(2011)=22f(2011) = \frac{\sqrt{2}}{2}. Consider f(0)=f(2011+(2011))=1f(2011)+1f(2011)f(0) = f(2011 + (-2011)) = \frac{1}{f(2011)} + \frac{1}{f(-2011)} which implies that 2=2+1f(2011)-\sqrt{2} = \sqrt{2} + \frac{1}{f(-2011)}. Thus, f(2011)=24f(-2011) = -\frac{\sqrt{2}}{4}.

But if we consider f(2011)=f(2011+0)=1f(2011)+1f(0)f(-2011) = f(-2011+0) = \frac{1}{f(-2011)} + \frac{1}{f(0)}, this means that 24=22+1f(0)-\frac{\sqrt{2}}{4} = -2\sqrt{2} + \frac{1}{f(0)}.
Thus, f(0)=227f(0) = \frac{2\sqrt{2}}{7} which is a contradiction. Thus for f(0)=2f(0) = -\sqrt{2}, f(2011)=2f(2011) = -\sqrt{2}.

So the possible values for f(2011)f(2011) are ±2\pm \sqrt{2}.

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.