A Mistake I Made

Algebra Level 2

TRUE or FALSE ?

For positive reals p , q p, q and b b with none of them equals 1 1 , p < q log b p < log b q . p< q \implies \log_{b}p < \log_{b}q .

False True

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1 solution

A counter-example: ( 1 2 ) 2 = 1 4 (\frac{1}{2})^2 = \frac{1}{4} and ( 1 2 ) 3 = 1 8 (\frac{1}{2})^3 = \frac{1}{8} .

Now, 1 8 < 1 4 \frac{1}{8} < \frac{1}{4} but log 1 2 1 8 = 3 log 1 2 1 4 = 2 \log_{\frac{1}{2}} {\frac{1}{8}} = 3 \not< \log_{\frac{1}{2}}{\frac{1}{4}} = 2 .


Generally, the statement in the problem description hold only when b > 1 b >1 ; even when any one or both of p p and q q can be 1 1 .

And the < < sign in the statement in the problem description gets replaced by > > , making the statement true, when b < 1 b <1 ; even when any one or both of p p and q q can be 1 1 .

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