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# If function f(x) = x² + 2bx + 4, then for any b, f(x) is the least whe

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Re: If function f(x) = x² + 2bx + 4, then for any b, f(x) is the least whe [#permalink]
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Hi Bunuel !!!!!

We can use differentiation approach too....I know this is out of scope but still we are bothered about getting right answer in least possible time....put df(x)/d(x) = 0.....d(x^2 + 2bx + 4)/d(x)=0....i.e. 2x + 2b = 0.......x=-b
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Re: If function f(x) = x² + 2bx + 4, then for any b, f(x) is the least whe [#permalink]
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Hi All,

This question can be solved by TESTing VALUES.

We're given the function f(X) = X^2 + 2BX + 4. We're told for ANY value of B, f(X) is LEAST when X = what value.

This means that we can choose ANY value we want for B and the answer will stay the SAME. Looking at the 5 answer choices, I don't want to use B=0 or B=2 (since those two possibilities would create duplicate answers); so I'll use...

B = 1

f(X) = X^2 + 2X + 4

Which of the following answers gives us the LEAST result when X = ......

A: X = -1 --> -1 - \sqrt{-3} = nonsenscial answer. Eliminate A.

B: X = -2 --> 4 - 4 + 4 = 4

C: X = 0 --> 0 + 0 + 4 = 4

D: X = -1 --> 1 - 2 + 4 = 3

E: X = -3 --> 9 - 6 + 4 = 7

GMAT assassins aren't born, they're made,
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Re: If function f(x) = x² + 2bx + 4, then for any b, f(x) is the least whe [#permalink]
Hi Bunuel !!!!!

We can use differentiation approach too....I know this is out of scope but still we are bothered about getting right answer in least possible time....put df(x)/d(x) = 0.....d(x^2 + 2bx + 4)/d(x)=0....i.e. 2x + 2b = 0.......x=-b

SEEMS TO BE THE BEST APPROACH.
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Re: If function f(x) = x² + 2bx + 4, then for any b, f(x) is the least whe [#permalink]
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Re: If function f(x) = x² + 2bx + 4, then for any b, f(x) is the least whe [#permalink]
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