Find the minimum value of the positive integer x satisfying the inequality 2 lo g 1 0 ( 2 x − 1 ) + lo g 1 0 x − 9 ≥ 1 .
Source: RMO
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Did in the same way!!!!!!!!!!!!!! Bro
The question is flawed as it does not specify the base to which the logarithm is specified. A more general solution would proceed thus :
2 l o g ( 2 x − 1 ) + l o g ( x − 9 ) ≥ 1
From this, it is clear that x > 9 for the square root to exist. Which translates to
l o g ( 2 x − 1 ) + l o g ( x − 9 ) ≥ 2
If it is assumed that the base is an unspecified positive number k , then the above inequality is equivalent to
( 2 x − 1 ) ( x − 9 ) ≥ k 2
or
2 x 2 − 1 9 x + 9 − k 2 ≥ 0
Which gives the solution as
x ≤ 4 1 ( 1 9 − 8 k 2 + 2 8 9 or x ≥ 4 1 ( 1 9 + 8 k 2 + 2 8 9
The first inequality is valid only if 1 9 > 8 k 2 + 2 8 9 or |(x<3). However, x > 9 . Hence, the second inequality would yield the solution as
x ≥ c e i l ( 4 1 ( 1 9 + 8 k 2 + 2 8 9 ) where c e i l ( y ) is used to denote the least integer greater than y
If natural logarithm is assumed then
x ≥ 1 0 .
However, if the base is assumed to be 10, the solution would be
x ≥ 1 3 .
If the base is assumed to be 100 (just for argument sake), then the solution would be
x ≥ 7 6
The base has (now) been specified!
Well, generally for logarithms, the base is taken as 1 0 when relating to Algebra and e when relating to Calculus problems (unless stated otherwise).
But still, only "generally", so, yes the perfect problem will be with base 10 and an okay(or general) problem with no base.
No base? How can you define logarithm with no base? :o
@Prasun Biswas – That was actually "no base"(with inverted quotation marks)!
@Kartik Sharma – Oh. You should've said that earlier. :P
@Prasun Biswas – irrespective of algebra or calculus ..... if base is not mentioned then it should be taken as base e not 10
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l o g ( 2 x − 1 ) + 2 l o g x − 9 ≥ 2
Using properties of logs we can write it as
l o g ( 2 x − 1 ) + l o g ( x − 9 ) ≥ 2
→ l o g ( 2 x − 1 ) ( x − 9 ) ≥ l o g 1 0 0
→ ( 2 x − 1 ) ( x − 9 ) ≥ 1 0 0
→ 2 x 2 − 1 9 x − 9 1 ≥ 0
→ ( 2 x + 7 ) ( x − 1 3 ) ≥ 0
We can also see that x can't be less than or equal to 2 1 because log for -ve no.s is not defined.
By this x ∈ [ 1 3 , ∞ )
→ ∣ x ∣ m i n = 1 3