SOLUTION: Solve for x: log(subscript5)(x)=-2. thanks.

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Question 202239: Solve for x: log(subscript5)(x)=-2. thanks.
Answer by Theo(13342) About Me  (Show Source):
You can put this solution on YOUR website!
this says:
log%285%2Cx%29+=+-2
because logarithms are inverse of exponents, this formula can only be true if and only if...
5%5E%28-2%29+=+x
which should be the same as:
1%2F5%5E2+=+x+
since 5%5E2 = 25, this becomes:
(1/25) = x
The answer should be (1/25)
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to prove this is true, substitute (1/25) for x and solve.
log%285%2C%281%2F25%29%29+=+-2+
is the same as:
log%285%2C1%29+-+log%285%2C25%29+=+-2
to solve using your calculator, you can convert the base 5 to the base 10 or to the natural log base.
let's try the base 10.
log%285%2C1%29+=+log%2810%2C1%29%2Flog%2810%2C5%29+ = 0/.698..... = 0
log%285%2C25%29+=+log%2810%2C25%29+%2F+log%2810%2C5%29+ = 2
so...
log%285%2C%281%2F25%29%29+=+log%285%2C1%29+-+log%285%2C15%29+=+-2+ which is true,
so x = (1/25) is a good answer.
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another way to prove it is to convert back to exponents by taking the inverse.
we start off with:
log%285%2C%281%2F25%29%29+=+-2
this is true if and only if:
5%5E%28-2%29+=+%281%2F25%29+
we'll cheat this time by using the calculator.
5%5E%28-2%29+=+.04+
.04 is the same as .04/1
multiply numerator and denominator by 25 (doesn't change the fraction), and we get
(.04*25)/(1*25) = (1/25), which is the answer we are looking for.
bottom line:
x = (1/25)
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