Multiplication of Power Tower

Algebra Level 3

( 1 + 2 2 0 ) ( 1 + 2 2 1 ) ( 1 + 2 2 2 ) ( 1 + 2 2 2015 ) \large \left( 1+{ 2 }^{ { 2 }^{ 0 } } \right) \left( 1+{ 2 }^{ { 2 }^{ 1 } } \right) \left( 1+{ 2 }^{ { 2 }^{ 2 } } \right) \cdots\left( 1+{ 2 }^{ { 2 }^{ 2015 } } \right)

If the expression above can be expressed as 2 A 1 2^A - 1 , find the value of log 2 A \log_2 A .


The answer is 2016.00.

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1 solution

Lam Yet Sin
Oct 20, 2015

Multiply the equation by ( 2 2 0 1 ) (2^{2^0}-1) . (Take note that 2 2 0 1 = 1 2^{2^0}-1=1 so it won't change the value of equation)

The equation now becomes

( 2 2 0 1 ) ( 2 2 0 + 1 ) ( 2 2 1 + 1 ) ( 2 2 2 + 1 ) . . . ( 2 2 2015 + 1 ) \left( { 2 }^{ { 2 }^{ 0 } }-1 \right) \left( { 2 }^{ { 2 }^{ 0 } }+1 \right) \left( { 2 }^{ { 2 }^{ 1 } }+1 \right) \left( { 2 }^{ { 2 }^{ 2 } }+1 \right) ...\left( { 2 }^{ { 2 }^{ 2015 } }+1 \right)

Take note that

( 2 a 1 ) ( 2 a + 1 ) = 2 2 a 1 (2^a-1)(2^a+1)=2^{2a}-1

2 2 k × 2 = 2 2 k + 1 2^{{2^k} \times 2}=2^{2^{k+1}}

Using these two identities,

( 2 2 0 1 ) ( 2 2 0 + 1 ) ( 2 2 1 + 1 ) ( 2 2 2 + 1 ) . . . ( 2 2 2015 + 1 ) \left( { 2 }^{ { 2 }^{ 0 } }-1 \right) \left( { 2 }^{ { 2 }^{ 0 } }+1 \right) \left( { 2 }^{ { 2 }^{ 1 } }+1 \right) \left( { 2 }^{ { 2 }^{ 2 } }+1 \right) ...\left( { 2 }^{ { 2 }^{ 2015 } }+1 \right)

= ( 2 2 1 1 ) ( 2 2 1 + 1 ) ( 2 2 2 + 1 ) . . . ( 2 2 2015 + 1 ) \left( { 2 }^{ { 2 }^{ 1 } }-1 \right) \left( { 2 }^{ { 2 }^{ 1 } }+1 \right) \left( { 2 }^{ { 2 }^{ 2 } }+1 \right)...\left( { 2 }^{ { 2 }^{ 2015 } }+1 \right)

= ( 2 2 2 1 ) ( 2 2 2 + 1 ) . . . ( 2 2 2015 + 1 ) \left( { 2 }^{ { 2 }^{ 2 } }-1 \right) \left( { 2 }^{ { 2 }^{ 2 } }+1 \right) ...\left( { 2 }^{ { 2 }^{ 2015 } }+1 \right)

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.

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= ( 2 2 2015 1 ) ( 2 2 2015 + 1 ) \left( { 2 }^{ { 2 }^{ 2015 } }-1 \right) \left( { 2 }^{ { 2 }^{ 2015 } }+1 \right)

= 2 2 2016 1 {2}^{{2}^{2016}}-1

A = 2 2016 A=2^{2016}

l o g 2 A = 2016 log_{2}{A} = \boxed{2016}

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