Logarithm Basics Part II

Algebra Level 5

log 0.3 ( x 1 ) < log 0.09 ( x 1 ) \displaystyle \log_{0.3} (x-1) < \log_{0.09}(x-1) The values of x x satisfying the above inequality are?


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(1,2) ( , 1 ) ( 2 , ) (-\infty,1) \cup (2,\infty) ( 1 , ) (1,\infty) ( 2 , ) (2,\infty)

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2 solutions

Sandeep Bhardwaj
May 6, 2015

l o g 0.3 ( x 1 ) < l o g 0.09 ( x 1 ) \displaystyle log_{0.3} (x-1) < log_{0.09}(x-1)

l o g ( x 1 ) l o g ( 0.3 ) < l o g ( x 1 ) l o g ( 0.09 ) \implies \dfrac{log(x-1)}{log(0.3)} < \dfrac{log(x-1)}{log(0.09)} ( l o g B A = l o g A l o g B ) \left( log_BA=\frac{logA}{logB} \right)

l o g ( x 1 ) l o g ( 0.3 ) < l o g ( x 1 ) 2 l o g ( 0.3 ) \implies \dfrac{log(x-1)}{log(0.3)} < \dfrac{log(x-1)}{2 \cdot log(0.3)}

l o g ( x 1 ) 2 > l o g ( x 1 ) \implies \displaystyle log (x-1)^2> log(x-1) (Inequality reversed...think : why ???)

x 2 2 x + 1 > x 1 \implies x^2-2x+1>x-1

x 2 3 x + 2 > 0 \implies x^2-3x+2>0

x ( , 1 ) ( 2 , ) \implies x \in (-\infty,1) \cup (2,\infty) . . . . . . . . . . . . . ( 1 ) \quad \quad \quad .............(1)

But taking domain of l o g log into consideration :

x 1 > 0 x-1>0

x > 1 \implies x>1 . . . . . . . . . . . . ( 2 ) \quad \quad \quad ............(2)

So combining ( 1 ) & ( 2 ) (1) \& (2) , the values of x x satisfying the above inequality are ( 2 , ) \boxed{(2,\infty)}

enjoy!

After getting ( x 1 ) 2 > x 1 (x-1)^{2} > x-1 we can divide both sides by x 1 x-1 as from the domain we know that x 1 > 0 x-1 >0 , this would straight away give us x 1 > 1 x-1>1 .

Shubhendra Singh - 6 years, 1 month ago

Sir I simply thought that

1 is not possible .

For x=2. They will be equal so the answer that follows is (2,infinity)

ashutosh mahapatra - 6 years, 1 month ago

A typo in the second step identity, it is actually :

l o g B A = l o g A l o g B log_{B} A = \dfrac{logA} {logB} .

Venkata Karthik Bandaru - 6 years, 1 month ago

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Thanks for figuring it out..I've corrected it. Karthik Venkata

Sandeep Bhardwaj - 6 years, 1 month ago

Why must the inequality be reversed when multiplied by a positive integer ??

Vaibhav Prasad - 6 years, 1 month ago

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l o g ( 0.3 ) log(0.3) is not a positive integer, it is a negative number.

Sandeep Bhardwaj - 6 years, 1 month ago

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ohhhh........that's it !!! Thank you

Vaibhav Prasad - 6 years, 1 month ago

But sir there is a property of log in which l o g 0.09 log_{0.09} can be directly written as 1 2 \frac{1}{2} l o g 0.3 log_{0.3} .So the inequality won't be reversed..

Naman Kapoor - 6 years, 1 month ago

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Still the inequality will be reversed on cancelling l o g 0.3 log_{0.3} from both sides. And if you don't cancel then :

If l o g a x 1 > l o g a x 2 log_ax_1>log_ax_2 and 0 < a < 1 0<a<1 , then x 1 < x 2 x_1<x_2 . So it is same as cancelling and reversing the inequality.

Sandeep Bhardwaj - 6 years, 1 month ago

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Oh I forgot that...

Naman Kapoor - 6 years, 1 month ago
Ravi Dwivedi
Jul 5, 2015

Moderator note:

Always be careful with logarithms of a base that is smaller than 1. Inequalities often end up working the other way.

Because of that, I tend to prefer to view log 1 n \log \frac{1}{n} as log n - \log n instead.

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