Inequalities is a big favorite of the GMAT test makers. Inequalities, especially presented as a data sufficiency question that appear in the GMAT test are many a times potential land mines.
Here is an inequality question in the problem solving format
Which of the following is correct if x is a real number and (x - 11)(x - 3) is negative?
A. x^2 + 5x + 6 < 0
B. x^2 + 5x + 6 > 0
C. 5 - x < 0
D. x - 5 < 0
E. 11 - x > 0
The question states that (x - 11)(x - 3) < 0
The solution to this inequality can be obtained as follows:
Case 1: When (x - 11) < 0 and (x - 3) > 0, the product will be negative.
Simplifying, we get x < 11 and x > 3. i.e., 3 < x < 11.
Case 2: When (x - 11) > 0 and (x - 3) < 0, the product will be negative
Simplifying, we get x > 11 and x < 3. This is an impossible solution as x cannot simultaneously be greater than 11 and less than 3. So, infeasible.
So, the solution set to the inequality is 3 < x < 11.
Now, let us look at the options.
x^2 + 5x + 6 < 0.
Factorizing the quadratic expression, we get (x + 2)(x + 3) < 0.
As we did with the expression in the question, we could get the solution set as -3 < x < -2. We know that the value of x as stated in the question lies between 3 and 11. So, this is not possible.
x^2 + 5x + 6 > 0.
The solution set will be exactly the opposite as that for choice A. so, x < -3 and x > -2. The question has values of x lying between 3 and 11, which satisfy the condition x > -2.
Hence, choice B is the correct answer.
Labels: GMAT Inequalities, GMAT Quadratic Equations