Maths Olympiad Prep

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Algebra Difficulty 7.4 National Olympiad, round 2 Prove it Philippines

Problem:
How many real numbers xx satisfy the equation
(x212x+20logx2)1+logx=x212x+201+log(1/x)? \left(\left|x^{2}-12 x+20\right|^{\log x^{2}}\right)^{-1+\log x}=\left|x^{2}-12 x+20\right|^{1+\log (1 / x)} ?

Solution

Solution:
Let A=x212x+20A = |x^2 - 12x + 20|.

The equation is:
(Alogx2)1+logx=A1+log(1/x) \left(A^{\log x^2}\right)^{-1 + \log x} = A^{1 + \log(1/x)}

First, A0A \geq 0 and x0x \neq 0 (since logx2\log x^2 is defined for x0x \neq 0).

Also, logx\log x is defined for x>0x > 0.

So, x>0x > 0.

Now, log(1/x)=logx\log(1/x) = -\log x, so 1+log(1/x)=1logx1 + \log(1/x) = 1 - \log x.

The equation becomes:
(Alogx2)1+logx=A1logx \left(A^{\log x^2}\right)^{-1 + \log x} = A^{1 - \log x}

Recall that Alogx2=A2logx=(Alogx)2A^{\log x^2} = A^{2 \log x} = (A^{\log x})^2.

So,
((Alogx)2)1+logx=A1logx \left((A^{\log x})^2\right)^{-1 + \log x} = A^{1 - \log x}
(Alogx)2(1+logx)=A1logx (A^{\log x})^{2(-1 + \log x)} = A^{1 - \log x}
A2(1+logx)logx=A1logx A^{2(-1 + \log x) \log x} = A^{1 - \log x}

If A=0A = 0, then x212x+20=0    x212x+20=0    x=2|x^2 - 12x + 20| = 0 \implies x^2 - 12x + 20 = 0 \implies x = 2 or x=10x = 10.

Check x=2x = 2:
A=0A = 0, logx=log2>0\log x = \log 2 > 0, logx2=2log2>0\log x^2 = 2 \log 2 > 0.

Left side: 0something0^{\text{something}} is 00 if exponent >0> 0, 11 if exponent =0= 0, undefined if exponent <0< 0.

But A=0A = 0, so 0something0^{\text{something}} is 00 if exponent >0> 0.

Right side: 01log20^{1 - \log 2}. 1log2<11 - \log 2 < 1, so exponent <1< 1.

But 0a=00^a = 0 for a>0a > 0, undefined for a0a \leq 0.

So, x=2x = 2 and x=10x = 10 do not give valid solutions.

Now, for A>0A > 0, A0A \neq 0:

Since A>0A > 0, AkA^k is defined for all real kk.

So, equate exponents:

If A1A \neq 1, then Ap=Aq    p=qA^p = A^q \implies p = q.

So,
2(1+logx)logx=1logx 2(-1 + \log x) \log x = 1 - \log x
Let y=logxy = \log x (so x>0x > 0):

2(y1)y=1y2(y - 1)y = 1 - y

2y22y=1y2y^2 - 2y = 1 - y

2y22y+y1=02y^2 - 2y + y - 1 = 0

2y2y1=02y^2 - y - 1 = 0

2y2y1=02y^2 - y - 1 = 0

Solve for yy:

y=1±1+84=1±34y = \frac{1 \pm \sqrt{1 + 8}}{4} = \frac{1 \pm 3}{4}

So y=1y = 1 or y=12y = -\frac{1}{2}.

So logx=1    x=10\log x = 1 \implies x = 10, logx=12    x=101/2=1/10\log x = -\frac{1}{2} \implies x = 10^{-1/2} = 1/\sqrt{10}.

Recall A=x212x+20A = |x^2 - 12x + 20|.

If A=1A = 1, then Ap=AqA^p = A^q for any p,qp, q.

So, A=1    x212x+20=1A = 1 \implies |x^2 - 12x + 20| = 1.

x212x+20=1x^2 - 12x + 20 = 1 or x212x+20=1x^2 - 12x + 20 = -1

x212x+19=0x^2 - 12x + 19 = 0

x=6±3619=6±17x = 6 \pm \sqrt{36 - 19} = 6 \pm \sqrt{17}

x212x+21=0x^2 - 12x + 21 = 0

x=6±3621=6±15x = 6 \pm \sqrt{36 - 21} = 6 \pm \sqrt{15}

But x>0x > 0.

So, possible xx values:
6+176 + \sqrt{17}, 6176 - \sqrt{17}, 6+156 + \sqrt{15}, 6156 - \sqrt{15}.

Check if these are positive:
174.12\sqrt{17} \approx 4.12, 64.12=1.88>06 - 4.12 = 1.88 > 0
153.87\sqrt{15} \approx 3.87, 63.87=2.13>06 - 3.87 = 2.13 > 0

So all four are positive.

Now, check if x=10x = 10 or x=1/10x = 1/\sqrt{10} gives A=1A = 1.

For x=10x = 10:
A=100120+20=0=01A = |100 - 120 + 20| = |0| = 0 \neq 1

For x=1/10x = 1/\sqrt{10}:
x2=1/10x^2 = 1/10, x212x+20=1/1012/10+20x^2 - 12x + 20 = 1/10 - 12/\sqrt{10} + 20

This is not ±1\pm 1.

So, x=10x = 10 and x=1/10x = 1/\sqrt{10} are not among the A=1A = 1 solutions.

Now, check if A>0A > 0 for x=1/10x = 1/\sqrt{10}:
x=1/10x = 1/\sqrt{10}, x2=1/10x^2 = 1/10, x212x+20=1/1012/10+20x^2 - 12x + 20 = 1/10 - 12/\sqrt{10} + 20

12/1012/3.163.812/\sqrt{10} \approx 12/3.16 \approx 3.8

So 1/103.8+200.13.8+20=16.31/10 - 3.8 + 20 \approx 0.1 - 3.8 + 20 = 16.3

So A>0A > 0.

So, x=1/10x = 1/\sqrt{10} is valid.

So, the solutions are:

- x=1/10x = 1/\sqrt{10}
- x=6+17x = 6 + \sqrt{17}
- x=617x = 6 - \sqrt{17}
- x=6+15x = 6 + \sqrt{15}
- x=615x = 6 - \sqrt{15}

Total: 55 real numbers xx.

Answer: 5\boxed{5}

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