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Algebra Level 3

Find the minimum value of the positive integer x x satisfying the inequality log 10 ( 2 x 1 ) 2 + log 10 x 9 1. \frac{\log_{10} (2x-1)}{2}+\log_{10} \sqrt{x-9} \geq 1.

Source: RMO


The answer is 13.

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2 solutions

Discussions for this problem are now closed

Shubhendra Singh
Jan 10, 2015

l o g ( 2 x 1 ) + 2 l o g x 9 2 log(2x-1)+2log\sqrt{x-9}\geq2

Using properties of logs we can write it as

l o g ( 2 x 1 ) + l o g ( x 9 ) 2 log(2x-1)+log({x-9})\geq2

l o g ( 2 x 1 ) ( x 9 ) l o g 100 \rightarrow log(2x-1)(x-9)\geq log100

( 2 x 1 ) ( x 9 ) 100 \rightarrow (2x-1)(x-9)\geq100

2 x 2 19 x 91 0 \rightarrow 2x^{2}-19x-91\geq0

( 2 x + 7 ) ( x 13 ) 0 \rightarrow(2x+7 )(x-13)\geq0

We can also see that x x can't be less than or equal to 1 2 \dfrac {1}{2} because log for -ve no.s is not defined.

By this x [ 13 , ) x\in [13 \ , \ \infty )

x m i n = 13 \huge{\rightarrow |x|_{min}=13}

Did in the same way!!!!!!!!!!!!!! Bro

Sudhir Aripirala - 6 years, 5 months ago

The question is flawed as it does not specify the base to which the logarithm is specified. A more general solution would proceed thus :

l o g ( 2 x 1 ) 2 + l o g ( x 9 ) 1 \frac{log(2x-1)}{2}+log(\sqrt{x-9}) \geq 1

From this, it is clear that x > 9 x>9 for the square root to exist. Which translates to

l o g ( 2 x 1 ) + l o g ( x 9 ) 2 log(2x-1)+log(x-9) \geq 2

If it is assumed that the base is an unspecified positive number k k , then the above inequality is equivalent to

( 2 x 1 ) ( x 9 ) k 2 (2x-1)(x-9) \geq k^2

or

2 x 2 19 x + 9 k 2 0 2x^2-19x+9-k^2 \geq 0

Which gives the solution as

x 1 4 ( 19 8 k 2 + 289 x \leq \frac{1}{4}(19-\sqrt{8k^2+289} or x 1 4 ( 19 + 8 k 2 + 289 x \geq \frac{1}{4}(19+\sqrt{8k^2+289}

The first inequality is valid only if 19 > 8 k 2 + 289 19>\sqrt{8k^2+289} or |(x<3). However, x > 9 x>9 . Hence, the second inequality would yield the solution as

x c e i l ( 1 4 ( 19 + 8 k 2 + 289 ) x \ge ceil\left(\frac{1}{4}(19+\sqrt{8k^2+289}\right) where c e i l ( y ) ceil(y) is used to denote the least integer greater than y y

If natural logarithm is assumed then

x 10 \huge{x\geq10} .

However, if the base is assumed to be 10, the solution would be

x 13 \huge{x \geq 13} .

If the base is assumed to be 100 (just for argument sake), then the solution would be

x 76 \huge{x \geq 76}

The base has (now) been specified!

Pi Han Goh - 6 years, 2 months ago

Well, generally for logarithms, the base is taken as 10 10 when relating to Algebra and e e when relating to Calculus problems (unless stated otherwise).

Prasun Biswas - 6 years, 5 months ago

But still, only "generally", so, yes the perfect problem will be with base 10 and an okay(or general) problem with no base.

Kartik Sharma - 6 years, 5 months ago

No base? How can you define logarithm with no base? :o

Prasun Biswas - 6 years, 5 months ago

@Prasun Biswas That was actually "no base"(with inverted quotation marks)!

Kartik Sharma - 6 years, 5 months ago

@Kartik Sharma Oh. You should've said that earlier. :P

Prasun Biswas - 6 years, 5 months ago

@Prasun Biswas irrespective of algebra or calculus ..... if base is not mentioned then it should be taken as base e not 10

Naveen Kanchi - 6 years, 4 months ago

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