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Bunuel
Is x < 0 ?

(1) x^5 < x^6

(2) 5^x > 6^x


We want to find if x lies to the left of 0 on a number line.

Statement 1

\(x^5 - x^6 < 0\)

\(x^5(1-x) < 0\)

We see that the product of two numbers is negative, there can be two possible cases

Case 1:

  • \(x^5 < 0\)
  • \((1 - x) > 0\)

As \(x^5 < 0; x < 0\)

1 - x > 0
- x > -1
x < 1

Combined, x < 0

Case 2:

  • \(x^5 > 0\)
  • \((1 - x) < 0\)

As \(x^5 >0; x > 0\)

(1-x) < 0
-x < -1
x > 1

Combined x > 1

So this equation hold true in two regions, x < 0 and x > 1.

As we're getting two regions, we can rule out statement 1 and eliminate A and D.

Statement 2

\(5^x > 6^x\)

\(6^x\) is positive, hence we can divide both sides on \(6^x\) without any change in the inequality sign

\((\frac{5}{6})^x\) > 1

Note that \(\frac{5}{6}\) will have value between 0 and 1, but when that decimal is raised to power x, the action makes the value greater than one. This can only happen when x is negative.

Hence x < 0.

Sufficient.

Option B
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