Tens, Tens everywhere

Algebra Level 4

Find the geometric mean of the numbers ( 1000000 10 + 100000 10 ) (1000000\sqrt{10}+100000\sqrt{10}) ( 100000 10 + 10000 10 ) (100000\sqrt{10}+10000\sqrt{10}) \vdots ( 1 10000 10 + 1 100000 10 ) \left(\dfrac{1}{10000}\sqrt{10}+\dfrac{1}{100000}\sqrt{10}\right) ( 1 100000 10 + 1 1000000 10 ) \left(\dfrac{1}{100000}\sqrt{10}+\dfrac{1}{1000000}\sqrt{10}\right)


The answer is 11.

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2 solutions

Daniel Liu
Jul 9, 2015

Notice what happens when we multiply the number 1 0 k + 1 10 + 1 0 k 10 10^{k+1}\sqrt{10}+10^{k}\sqrt{10} with 1 1 0 k + 1 10 + 1 1 0 k 10 \dfrac{1}{10^{k+1}}\sqrt{10}+\dfrac{1}{10^{k}}\sqrt{10} : ( 1 0 k + 1 10 + 1 0 k 10 ) ( 1 1 0 k + 1 10 + 1 1 0 k 10 ) = 10 + 100 + 1 + 10 = 121 (10^{k+1}\sqrt{10}+10^{k}\sqrt{10}) \left(\dfrac{1}{10^{k+1}}\sqrt{10}+\dfrac{1}{10^{k}}\sqrt{10}\right) = 10+100+1+10 = 121 Thus, the geometric mean of those two numbers is 121 = 11 \sqrt{121}=11 .

Since all of these numbers come in pairs, and the geometric mean of each pair is 11 11 , the geometric mean of all the numbers is also 11 \boxed{11} .

Nice problem!

Justin Dong - 5 years, 11 months ago

Impressive Problem!

Peter Michael - 5 years, 11 months ago

nice problem

Dev Sharma - 5 years, 6 months ago
Utkarsh Grover
Oct 7, 2015

the terms of the given expression can be simplified as /( 11\sqrt { 10 } \times { 10 }^{ 5 }\quad ;\quad 11\sqrt { 10 } \times { 10 }^{ 4 }\quad .............\quad 11\sqrt { 10 } \times { 10 }^{ -6 } /) . multiplying all these terms the above expression reduces to /( 11^{12} \times 10^{-6} \times 10^{6} /) which is /( 11^{12} /) . now taking twelfth root of /(11^{12} /) leaves us with /( \boxed{11} /) which is the answer

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