Problems need HardWork #10

Algebra Level 3

Find the sum of all values of x x such that

4 x + 4 1 / x = 18. \large 4^x + 4^{1/x} = 18.


The answer is 2.5.

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2 solutions

4 x + 4 1 x = 18 4 x + 4 1 x 18 = 0 \large{4^x+4^{\frac{1}{x}}=18 \Rightarrow 4^x+4^{\frac{1}{x}}-18=0 }

4 x + 4 1 x 16 2 = 0 \large{4^x+4^{\frac{1}{x}}-16-2 =0}

4 x 16 = 0 x = 2 \large{\Rightarrow 4^x-16=0 \Rightarrow x=\boxed{2}}

or

4 1 x 2 = 0 2 2 × 1 x = 2 1 x = 1 2 \large{4^{\frac{1}{x}}-2=0 \Rightarrow 2^{2\times\frac{1}{x}}=2^1 \Rightarrow x=\boxed{\frac{1}{2}}}

Take 2 + 1 2 = 5 2 \large{2+ \frac{1}{2}=\boxed{\color{#D61F06}{\frac{5}{2}}}}

It's like 4 x , 4 1 / x > 0 Hence, 4 x = 2 and 4 1 / x = 16 x = 1 2 O R 4 x = 16 and 4 1 / x = 2 x = 2 4^x, 4^{1/x} > 0\\ \text{Hence, } 4^x = 2~ \text{and}~ 4^{1/x} = 16 \Rightarrow x = \dfrac{1}{2}\\ OR~ 4^x = 16~ \text{and}~ 4^{1/x} = 2 \Rightarrow x = 2\\

Kishore S. Shenoy - 5 years, 9 months ago

sir, why did you choose 4^x - 16 = 0

Dev Sharma - 5 years, 9 months ago

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Dear sir, you may choose 4^x-2=0 => x=1/2

And as same as the other pair.

Any combination is possible to find answer, which receives the same x.

Adam Phúc Nguyễn - 5 years, 9 months ago

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Thanks sir.

No sir please, I am just a student.

Dev Sharma - 5 years, 9 months ago

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@Dev Sharma Me too :D I'm same age as you.

Adam Phúc Nguyễn - 5 years, 9 months ago

Your solutions are excellent.

See my problems in my set.

Dev Sharma - 5 years, 9 months ago

Yup , that's it

Kishore S. Shenoy - 5 years, 9 months ago

How do you prove that these are the only solutions?

Abhinav Raichur - 5 years, 9 months ago

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by drawing graph

Dev Sharma - 5 years, 9 months ago

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okay ... thanks :)

Abhinav Raichur - 5 years, 9 months ago

Was somewhat easier than? Isn't?

Aparna Kalbande - 5 years, 6 months ago
Dev Sharma
Aug 22, 2015

Clearly, x = 2 or 1/2.

So, 2 + 0.5 = 2.5

true. Nice question. Is that hardwork? This problem doesnt need hardwork. Good job @Dev Sharma .

Nelson Mandela - 5 years, 9 months ago

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sir, its part of my set, Problems nee HardWork. Please see that set.

Dev Sharma - 5 years, 9 months ago

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okay @Dev Sharma

Nelson Mandela - 5 years, 9 months ago

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@Nelson Mandela sir, have you seen that set?

Dev Sharma - 5 years, 9 months ago

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@Dev Sharma I am not sir, I am only 15 years old and yes the questions in that set are very good. Keep it up @Dev Sharma .

Nelson Mandela - 5 years, 9 months ago

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