Awli wrote:
Is x > 0 ?
1. (x^3) < (x^4)
2. (x^3) < (x^5)
An alternate solution to this question is by using
The Wavy Line method Analyzing Statement 1 first:\(x^3\) < \(x^4\)
Subtracting both sides of an inequality with the same number doesn't affect the sign of inequality. So, let's subtract both sides of the inequality with \(x^3\). We get:
0 < \(x^4\) - \(x^3\)
This inequality can also be written as:
\(x^4\) - \(x^3\) > 0
Factorizing the expression on the Left Hand Side:
\(x^3\)(x - 1) > 0
The value of the LHS will be zero for x = 0 and x = 1. So, 0 and 1 are known as the
Zero Points of this expression.
Let's plot these two points on the number line. Then, starting from the
top right corner, let's draw a wavy line that passes through these two points.
The given expression will be
> 0 (that is,
positive) in the regions where the Wavy Line is
above the number line.
And, the given expression will be
< 0 in the regions where the Wavy Line is
below the number line.
So, we see that the inequality given in St. 1 (\(x^3\) < \(x^4\)) will hold for x < 0 and for x > 1.
Therefore, we cannot say for sure if x > 0 or not.
So,
Statement 1 alone is not sufficient.Let's now analyze Statement 2 alone:\(x^3\) < \(x^5\)
Subtracting both sides of this inequality with \(x^3\), we get:
0 < \(x^5\) - \(x^3\)
This inequality can also be written as:
\(x^5\) - \(x^3\) > 0
Factorizing the expression on the Left Hand Side:
\(x^3\)\((x^2 - 1)\) > 0
This can be further factorized as:
\(x^3\) (x - 1)(x+1) > 0
The value of the LHS will be zero for x = -1, x = 0 and x = 1. So, -1, 0 and 1 are the
Zero Points of this expression.
Let's plot these three points on the number line. Then, starting from the
top right corner, we'll draw a wavy line that passes through these two points.
So, we see that the inequality given in St. 2 (\(x^3\) < \(x^5\)) will hold for -1 < x < 0 and for x > 1.
Therefore, we cannot say for sure if x > 0 or not.
So, Statement 2 alone is not sufficient.Let's now combine the information from the two statements:From Statement 1, we inferred that:
Either x < 0 Or x > 1
From Statement 2, we inferred that:
Either -1 < x < 0 Or x > 1
By combining these two statements, we get:
Either -1 < x < 0 Or x > 1
Therefore, we still have not been able to determine for sure if x is positive.
So, the correct answer is E.
Hope this helped!
- Japinder