ExpertsGlobal5
Which of the following is (are) always correct?
I. If \(x^2\) is greater than \(x\), \(x^3\) is greater than \(x^2\).
II. If \(x\) is greater than \(\frac{1}{x}\), \(x^2\) is greater than \(\frac{1}{x}\).
III. If \(x^3\) is greater than \(x^2\), \(x^4\) is greater than \(x^3\).
A. I only
B. II only
C. III only
D. II and III only
E. I, II, and III
Let's analyze each statement individually.
Statement I: If \(x^2 > x\), then \(x^3 > x^2\)First, determine the range of \(x\) for which \(x^2 > x\).
\(x^2 - x > 0\)
\(x(x - 1) > 0\)
This inequality holds true when \(x > 1\) OR \(x < 0\).
Now, let's test a value from the range \(x < 0\).
Let \(x = -2\).
Condition: \((-2)^2 > -2 \implies 4 > -2\) (True).
Result check: Is \((-2)^3 > (-2)^2\)?
\(-8 > 4\) (FALSE).
Since it fails for negative numbers, Statement I is
not always correct.
Statement II: If \(x > \frac{1}{x}\), then \(x^2 > \frac{1}{x}\)First, solve \(x > \frac{1}{x}\).
\(x - \frac{1}{x} > 0\)
\(\frac{x^2 - 1}{x} > 0\)
The critical points are -1, 0, and 1. Testing the intervals, this inequality is true when:
1. \(-1 < x < 0\)
2. \(x > 1\)
Now check if \(x^2 > \frac{1}{x}\) holds in these two regions:
Region 1 (\(x > 1\)):Here, \(x\) is positive. \(x^2\) is positive and \(\frac{1}{x}\) is a positive fraction less than 1. A number greater than 1 squared is definitely greater than a proper fraction. (True).
Region 2 (\(-1 < x < 0\)):Here, \(x\) is negative.
\(x^2\) is
positive.
\(\frac{1}{x}\) is
negative.
A positive number is always greater than a negative number. So, \(x^2 > \frac{1}{x}\) is always True.
Statement II is
always correct.
Statement III: If \(x^3 > x^2\), then \(x^4 > x^3\)First, solve \(x^3 > x^2\).
Since \(x^2\) is always positive (for \(x \neq 0\)), we can divide both sides by \(x^2\) without flipping the inequality sign.
\(\frac{x^3}{x^2} > \frac{x^2}{x^2}\)
\(x > 1\).
So the condition implies that \(x\) must be greater than 1.
Now check the result \(x^4 > x^3\) given that \(x > 1\).
Divide by \(x^3\) (which is positive since \(x > 1\)):
\(x > 1\).
This is consistent with the condition.
Statement III is
always correct.
Conclusion: Statements II and III are correct.
Answer: D