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Re: If (-5)^(4x) = 5^(9 + x) and x is an integer, what is the value of x ? [#permalink]
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Carcass wrote:
If \((-5)^{(4x)} = 5^{(9 + x)}\) and x is an integer, what is the value of x ?

A. 5
B. 4
C. 3
D. 2
E. 1


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Question part of the project GRE Quantitative Reasoning Daily Challenge - (2021) EDITION
GRE - Math Book


The important concept here is: (negative number)^(even integer) = some positive value
Examples
(-5)^2 = 25
(-3)^4 = 81
(-1)^10 = 1
(-2)^6 = 64

Also notice that:
(-5)^2 = (5)^2 = 25
(-3)^4 = (3)^4 = 81
(-1)^10 = (1)^10 = 1
(-2)^6 = (2)^6 = 64
The same can be said for ANY negative value raised to an EVEN power

Now onto the question.............
Since 4x is EVEN for any integer x, we can write:
(-5)^(4x) = (5)^(4x)

So, we can write: 5^(4x) = 5^(9 + x)
Since we now have the base, we can conclude that 4x = 9 + x
Solve to get: x = 3

Answer: C
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