Multiply the Powers!

Suppose, a natural number x x and its any 2 factors, m m and n n such that x , m , n Z + x, m, n ∈ { ℤ }^{ + } (Positive Integers) and m × n = x m \times n=x , then select the option which is always true.

m n < 2 m n \large { m }^{ n }<{ 2 }^{ mn } m n 2 m n { m }^{ n }\le { 2 }^{ mn } None of the above options is always true. m n 2 m n { m }^{ n }\ge { 2 }^{ mn } m n > 2 m n \large { m }^{ n }>{ 2 }^{ mn }

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3 solutions

@Siddharth Chakravarty ,Is it correct?

A Former Brilliant Member - 8 months, 3 weeks ago

Yes it is, simple solution 🙂

Siddharth Chakravarty - 8 months, 3 weeks ago

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Thank you Siddharth

A Former Brilliant Member - 8 months, 3 weeks ago
Aryan Sanghi
Sep 15, 2020

Let L = m n and R = 2 m n L = m^n \text{ and } R = 2^{mn}

log 2 L = log 2 ( m n ) and log 2 R = log 2 ( 2 m n ) \log_2L = \log_2(m^n) \text{ and } \log_2R = \log_2(2^{mn})

log 2 L = n log 2 ( m ) and log 2 R = m n log 2 ( 2 ) \log_2L = n\log_2(m) \text{ and } \log_2R = mn\log_2(2)

log 2 L = n log 2 ( m ) and log 2 R = m n \log_2L = n\log_2(m) \text{ and } \log_2R = mn

Now, for all m Z + m \in \Z^+ ,

log 2 m < m \log_2m \lt m

n log 2 m < n m ( as n > 0 ) n\log_2m \lt nm \ldots (\text{as } n \gt 0)

log 2 L < log 2 R \log_2L \lt \log_2R

Now, as log 2 x \log_2x is continuous increasing function, so if log 2 x 1 < log 2 x 2 \log_2x_1 \lt \log_2x_2 , then x 1 < x 2 x_1 \lt x_2 . Therefore

L < R L \lt R

m n < 2 m n \color{#3D99F6}{\boxed{m^n \lt 2^{mn}}}

Great solution, and thanks for posting it😊

Siddharth Chakravarty - 9 months ago

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Nice question too. Thanku for sharing it. :)

Aryan Sanghi - 9 months ago

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Try solving it using no. of elements in a power set of a cartesian product of 2 sets and the no. of functions mapped over the cartesian product, the answer is obvious then.

Siddharth Chakravarty - 9 months ago

Only prime numbers have two factors whose product equals the number itself . Hence x x must be a prime number. It has factors 1 1 and x x itself, such that m n = 1 m^n=1 or x x , of which x 2 x\geq 2 is the larger.

2 m n = 2 x 2^{mn}=2^x , which is greater than x x for all values of x 2 x\geq 2 .

Sir you have misinterpreted the problem, the problem states we can take any 2 factors of the number, it does not say that it has only 2 factors. Although this is just a special case of the whole thing.

Siddharth Chakravarty - 8 months, 4 weeks ago

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