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If f(x) = 4x^2 + px, what is the minimum value of f(x)?

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by Max@Math Revolution » Wed Jul 04, 2018 12:37 am

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Answers

A

B

C

D

E

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[GMAT math practice question]

If f(x) = 4x^2 + px, what is the minimum value of f(x)?

1) f(1) = -4
2) f(2) = 0
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by GMATGuruNY » Wed Jul 04, 2018 1:49 am
Max@Math Revolution wrote:[GMAT math practice question]

If f(x) = 4x^2 + px, what is the minimum value of f(x)?

1) f(1) = -4
2) f(2) = 0
To determine the minimum value of the graph, we need to know the value of p.
Question stem, rephrased:
What is the value of p?

Statement 1:
Substituting x=1 and f(x)=−4 into f(x) = 4x² + px, we get:
-4 = 4(1)² + p(1)
Since we can solve for p, SUFFICIENT.

Statement 2:
Substituting x=2 and f(x)=0 into f(x) = 4x² + px, we get:
0 = 4(2)² + p(2)
Since we can solve for p, SUFFICIENT.

The correct answer is D.
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by Max@Math Revolution » Fri Jul 06, 2018 12:28 am
=>
Forget conventional ways of solving math questions. For DS problems, the VA (Variable Approach) method is the quickest and easiest way to find the answer without actually solving the problem. Remember that equal numbers of variables and independent equations ensure a solution.

Since we have 1 variable (p) and 0 equations, D is most likely to be the answer. So, we should consider each of the conditions on their own first.

Condition 1)
Since f(1) = -4, we have 4 + p = -4 and p = -8.
f(x) = 4x^2 -8x = 4( x^2 - 2x + 1 - 1 ) = 4(x-1)^2 - 4.
The minimum of f(x) is -4.
Condition 1) is sufficient.

Condition 2)
Since f(2) = 0, we have 16 + 2p = 0 and p = -8.
f(x) = 4x^2 -8x = 4( x^2 - 2x + 1 - 1 ) = 4(x-1)^2 - 4.
The minimum of f(x) is -4.
Condition 2) is sufficient.

Therefore, D is the answer.

Answer: D

Note: Tip 1) of the VA method states that D is most likely to be the answer if condition 1) gives the same information as condition 2).


If the original condition includes "1 variable", or "2 variables and 1 equation", or "3 variables and 2 equations" etc., one more equation is required to answer the question. If each of conditions 1) and 2) provide an additional equation, there is a 59% chance that D is the answer, a 38% chance that A or B is the answer, and a 3% chance that the answer is C or E. Thus, answer D (conditions 1) and 2), when applied separately, are sufficient to answer the question) is most likely, but there may be cases where the answer is A,B,C or E.
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