Lots of 32

Algebra Level 2

32 3 32 3 4 32 4 5 32 9 10 = ? \LARGE \sqrt[3]{ \sqrt{32}} \sqrt[4]{ \sqrt[3]{32}} \sqrt[5]{ \sqrt[4]{32}} \cdots \sqrt[10]{ \sqrt[9]{32}} = \ ?


The answer is 4.

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6 solutions

n = 1 8 [ ( 32 ) 1 n + 1 ] 1 n + 2 = n = 1 8 [ ( 2 5 ) 1 n + 1 ] 1 n + 2 = n = 1 8 [ 2 5 n + 1 ] 1 n + 2 = n = 1 8 2 5 ( n + 1 ) ( n + 2 ) = 2 n = 1 8 5 ( n + 1 ) ( n + 2 ) = 2 5 n = 1 8 ( 1 n + 1 1 n + 2 ) = 2 5 ( 1 2 1 10 ) = 2 2 = 4 \begin{aligned} \prod_{n=1}^8 {\left[ \left( 32 \right)^{\frac{1}{n+1}} \right]^{\frac{1}{n+2}}} & = \prod_{n=1}^8 {\left[ \left( 2^5 \right) ^{\frac {1} {n+1}} \right]^{\frac{1}{n+2}}} = \prod_{n=1}^8 {\left[2^{\frac {5} {n+1}} \right]^{\frac{1}{n+2}}} = \prod_{n=1}^8 {2^{\frac {5} {(n+1)(n+2)}}} \\ & = 2^{\sum_{n=1}^8 {\frac {5} {(n+1)(n+2)}}} = 2^{5 \sum_{n=1}^8 {\left( \frac {1} {n+1} - \frac {1} {n+2}\right)}} \\ & = 2^{5 \left( \frac{1}{2} - \frac{1}{10} \right)} = 2^2 = \boxed{4} \end{aligned}

@Chew-Seong Cheong , we really liked your comment, and have converted it into a solution. If you subscribe to this solution, you will receive notifications about future comments.

Brilliant Mathematics Staff - 6 years, 1 month ago

32 3 32 3 4 32 4 5 32 9 10 \LARGE \sqrt[3]{ \sqrt{32}} \sqrt[4]{ \sqrt[3]{32}} \sqrt[5]{ \sqrt[4]{32}} \cdots \sqrt[10]{ \sqrt[9]{32}} 3 2 1 2 × 1 3 3 2 1 3 × 1 4 3 2 1 4 × 1 5 . . . 3 2 1 9 × 1 10 \LARGE 32^{\frac{1}{2} \times \frac{1}{3}} 32^{\frac{1}{3} \times \frac{1}{4}} 32^{\frac{1}{4} \times \frac{1}{5}} ... 32^{\frac{1}{9} \times \frac{1}{10}} 3 2 1 2.3 + 1 3.4 + 1 4.5 + . . . + 1 9.10 \LARGE 32^{\frac {1}{2.3} + \frac {1}{3.4} + \frac {1}{4.5} +... +\frac{1}{9.10}} 3 2 2 5 \LARGE 32^{\frac{2}{5}} 4 \LARGE \boxed{4}

Moderator note:

As Jake Lai has pointed out. It's better to mention that you used telescoping sum because it's the key to simplify the calculation.

Might help to show that it's a telescoping sum.

Jake Lai - 6 years, 1 month ago
Quintessence Anx
May 4, 2015

The fractional exponents are: 1 3 1 2 , 1 4 1 3 , . . . 1 n 1 n 1 , . . . 1 10 1 9 \frac{1}{3}*\frac{1}{2},\frac{1}{4}*\frac{1}{3},...\frac{1}{n}*\frac{1}{n-1},...\frac{1}{10}*\frac{1}{9} .

This makes the series: n = 3 10 ( 3 2 1 n ( n 1 ) ) \prod_{n=3}^{10}(32^\frac{1}{n(n-1)})

Because a m a n = a m n a^m*a^n=a^{mn} , this in turn is: n = 3 10 ( 3 2 1 n ( n 1 ) ) = 3 2 n = 3 10 ( 1 n ( n 1 ) ) \prod_{n=3}^{10}(32^\frac{1}{n(n-1)}) = 32^{\sum_{n=3}^{10}(\frac{1}{n(n-1)})}

The series n = 2 m ( 1 n ( n 1 ) ) = m 1 m \sum_{n=2}^{m}(\frac{1}{n(n-1)}) = \frac{m-1}{m} , but since we're starting at n = 3 n=3 , we'll need to subtract the value where n = 2 n=2 . (A quick substitution shows this to be 1 2 \frac{1}{2} .)

So: n = 3 m ( 1 n ( n 1 ) ) = n = 2 m ( 1 n ( n 1 ) ) 1 2 = 10 1 10 1 2 = 9 10 1 2 = 4 10 = 2 5 \sum_{n=3}^{m}(\frac{1}{n(n-1)}) = \sum_{n=2}^{m}(\frac{1}{n(n-1)}) - \frac{1}{2} = \frac{10-1}{10} - \frac{1}{2} \\ = \frac{9}{10} - \frac{1}{2} = \frac{4}{10} = \frac{2}{5}

Thus: n = 3 10 ( 3 2 1 n ( n 1 ) ) = 3 2 n = 3 10 ( 1 n ( n 1 ) ) = 3 2 2 5 = 2 5 2 5 = 2 2 = 4 \prod_{n=3}^{10}(32^\frac{1}{n(n-1)}) = 32^{\sum_{n=3}^{10}(\frac{1}{n(n-1)})} = 32^{\frac{2}{5}} = {2^5}^{\frac{2}{5}} = 2^2 = 4

Rajen Kapur
May 3, 2015

Sum of the exponents of 32 are 1 2.3 + 1 3.4 + 1 4.5 + . . . . . . + 1 9.10 \frac {1}{2.3} + \frac {1}{3.4} + \frac {1}{4.5} +...... +\frac{1}{9.10} which adds up to 1 2 1 10 \frac{1}{2} - \frac{1}{10} = 2 5 \frac {2}{5} as exponent of 32. Answer = 4

Similar method as Rajen Kapur

n = 1 8 [ ( 32 ) 1 n + 1 ] 1 n + 2 = n = 1 8 [ ( 2 5 ) 1 n + 1 ] 1 n + 2 = n = 1 8 [ 2 5 n + 1 ] 1 n + 2 = n = 1 8 2 5 ( n + 1 ) ( n + 2 ) = 2 n = 1 8 5 ( n + 1 ) ( n + 2 ) = 2 5 n = 1 8 ( 1 n + 1 1 n + 2 ) = 2 5 ( 1 2 1 10 ) = 2 2 = 4 \begin{aligned} \prod_{n=1}^8 {\left[ \left( 32 \right)^{\frac{1}{n+1}} \right]^{\frac{1}{n+2}}} & = \prod_{n=1}^8 {\left[ \left( 2^5 \right) ^{\frac {1} {n+1}} \right]^{\frac{1}{n+2}}} = \prod_{n=1}^8 {\left[2^{\frac {5} {n+1}} \right]^{\frac{1}{n+2}}} = \prod_{n=1}^8 {2^{\frac {5} {(n+1)(n+2)}}} \\ & = 2^{\sum_{n=1}^8 {\frac {5} {(n+1)(n+2)}}} = 2^{5 \sum_{n=1}^8 {\left( \frac {1} {n+1} - \frac {1} {n+2}\right)}} \\ & = 2^{5 \left( \frac{1}{2} - \frac{1}{10} \right)} = 2^2 = \boxed{4} \end{aligned}

Chew-Seong Cheong - 6 years, 1 month ago
Mohd Sasa
May 7, 2015

It goes like that: A=sqrt of 3(sqrt(32)) sqrt of 4(sqrt of 3(32)) ...* sqrt of 10(sqrt of 9(32)) =(32^(1/2 * 1/3)) (32^(1/3 * 1/4)) ...*(32^(1/9 * 1/10)) =32^( 1/2 * 1/3 + 1/3 * 1/4 +...+ 1/9 * 1/10 )=32^X

X=1/2 * 1/3 + 1/3 * 1/4 + ... + 1/9 * 1/10 =1/2 - 1/3 + 1/3 - 1/4 + ... + 1/9 - 1/10 =1/2 - 1/10 = 2/5

A=32^(2/5)=4 Nice one.

How did you get: 1/2 * 1/3 + 1/3 * 1/4 + ... + 1/9 * 1/10 equals this: 1/2 - 1/3 + 1/3 - 1/4 + ... + 1/9 - 1/10

Joseph Li - 6 years, 1 month ago
Dhanya Jose
May 8, 2015

32^(1/2.3+1/3.4+1/4.5+.................+1/9.10) 1/2.3=1/2-1/3 1/3.4=1/3-1/4 so (1/2-1/3)+(1/3-1/4)+(1/4-1/5).................(1/9-1/10) =1/2-1/10 =====>2/5 32^(2/5) =======>((32)^1/5)^2===========>2^2=4

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