lo g 0 . 3 ( x − 1 ) < lo g 0 . 0 9 ( x − 1 ) The values of x satisfying the above inequality are?
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After getting ( x − 1 ) 2 > x − 1 we can divide both sides by x − 1 as from the domain we know that x − 1 > 0 , this would straight away give us x − 1 > 1 .
Sir I simply thought that
1 is not possible .
For x=2. They will be equal so the answer that follows is (2,infinity)
A typo in the second step identity, it is actually :
l o g B A = l o g B l o g A .
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Thanks for figuring it out..I've corrected it. Karthik Venkata
Why must the inequality be reversed when multiplied by a positive integer ??
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l o g ( 0 . 3 ) is not a positive integer, it is a negative number.
But sir there is a property of log in which l o g 0 . 0 9 can be directly written as 2 1 l o g 0 . 3 .So the inequality won't be reversed..
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Still the inequality will be reversed on cancelling l o g 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 and 0 < a < 1 , then x 1 < x 2 . So it is same as cancelling and reversing the inequality.
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 lo g n 1 as − lo g n instead.
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l o g 0 . 3 ( x − 1 ) < l o g 0 . 0 9 ( x − 1 )
⟹ l o g ( 0 . 3 ) l o g ( x − 1 ) < l o g ( 0 . 0 9 ) l o g ( x − 1 ) ( l o g B A = l o g B l o g A )
⟹ l o g ( 0 . 3 ) l o g ( x − 1 ) < 2 ⋅ l o g ( 0 . 3 ) l o g ( x − 1 )
⟹ l o g ( x − 1 ) 2 > l o g ( x − 1 ) (Inequality reversed...think : why ???)
⟹ x 2 − 2 x + 1 > x − 1
⟹ x 2 − 3 x + 2 > 0
⟹ x ∈ ( − ∞ , 1 ) ∪ ( 2 , ∞ ) . . . . . . . . . . . . . ( 1 )
But taking domain of l o g into consideration :
x − 1 > 0
⟹ x > 1 . . . . . . . . . . . . ( 2 )
So combining ( 1 ) & ( 2 ) , the values of x satisfying the above inequality are ( 2 , ∞ )
enjoy!