Algebra 3 - by Vlad Vasilescu (W)

Algebra Level 3

( 5 + 2 6 ) x + ( 5 2 6 ) x = 10 \large \left(\sqrt{5+2\sqrt{6}}\right)^x + \left(\sqrt{5-2\sqrt{6}}\right)^x = 10

The equation above has 2 roots. Find the positive root to 2 decimal places.


The answer is 2.00.

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1 solution

Vlad Vasilescu
Nov 25, 2016

We can see that ( 5 + 2 6 ) x ( 5 2 6 ) x = 1 (\sqrt{5+2\sqrt{6}})^x \cdot (\sqrt{5-2\sqrt{6}})^x = 1

We say ( 5 + 2 6 ) x = t ( 5 2 6 ) x = 1 t (\sqrt{5+2\sqrt{6}})^x = t \Rightarrow (\sqrt{5-2\sqrt{6}})^x = \frac{1}{t}

We solve for t in t + 1 t = 10 t 1 = 5 + 2 6 t + \frac{1}{t} = 10 \Rightarrow t_{1} = 5 + 2\sqrt{6} and t 2 = 5 2 6 t_{2} = 5 - 2\sqrt{6 }

For t = 5 + 2 6 t = 5 + 2\sqrt{6} we get x=2 and for t = 5 2 6 t = 5 - 2\sqrt{6} we get x=-2 .

Then , our answer will be 2 .

If it is precisely 2, why ask an answer in 2 decimals? I really started doubting when I found 2 as the answer because of the question stating 2 decimals.

Peter van der Linden - 4 years, 6 months ago

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Iti's about self confidence. When you know you got the correct answer you skip the other details .

Vlad Vasilescu - 4 years, 6 months ago

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Hehe yeah I know, but still... maybe I have been listening to my students to much ;)

Peter van der Linden - 4 years, 6 months ago

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