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If there is exactly one root of the equation x^2 + ax + b

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If there is exactly one root of the equation x^2 + ax + b [#permalink] New post 03 Mar 2012, 18:14
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If there is exactly one root of the equation x^2 + ax + b, where a and b are positive constants, what is b in terms of a?

A. a/2
B. a
C. 3a/2
D. a^2/2
E. a^2/4

I'm not sure how to solve this problem. It take me almost five minutes of brainstorming but nothing.

The only thing on how I 'm triyng to attack the same is : (x+b)^2 whre the only root is x=-b.

:(
[Reveal] Spoiler: OA

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Re: If there is exactly one root of the equation x^2 + ax + b [#permalink] New post 03 Mar 2012, 21:52
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If there is only one root then discriminant b^2-4ac=0 i.e both roots are equal

so here from the equation we get a^2-4b=0 so solving we get b=a^2/4

Hope it helps

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Re: If there is exactly one root of the equation x^2 + ax + b [#permalink] New post 03 Mar 2012, 22:14
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carcass wrote:
If there is exactly one root of the equation x^2 + ax + b, where a and b are positive constants, what is b in terms of a?

A. a/2
B. a
C. 3a/2
D. a^2/2
E. a^2/4

I'm not sure how to solve this problem. It take me almost five minutes of brainstorming but nothing.

The only thing on how I 'm triyng to attack the same is : (x+b)^2 whre the only root is x=-b.

:(


Yes, you can solve it this way: x^2+ax+b=0 will have only one root if it can be factored as (x+n)^2=0, in this case the root will be x=-n. (x+n)^2=x^2+2nx+n=0 --> a=2n and b=n^2. Now, since n=\frac{a}{2}, then b=(\frac{a}{2})^2=\frac{a^2}{4}.

Answer: E.

Or: a quadratic function is ax^2+bx+c=0 will have only one root (only one intercept with x-axis) if discriminant is zero, so when discriminant=b^2-4ac=0.

For given expression x^2+ax+b=0 discriminant is a^2-4b, so it must equal to zero: a^2-4b=0 --> b=\frac{a^2}{4}.

Answer: E.

Or: try number plugging, x^2+ax+b=0 will have only one root if it can be factored for example as (x+4)^2=0 --> x^2+8x+16=0 --> a=8 and b=16. Now, plug a=8 in the answer choices and see which one gives b=16, only answer choice E works.

Answer: E.

Note that for plug-in method it might happen that for some particular number(s) more than one option may give "correct" answer. In this case just pick some other numbers and check again these "correct" options only. For example if you pick (x+2)^2=0 then you get two "correct" options B and E.

Hope it helps.
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Re: If there is exactly one root of the equation x^2 + ax + b [#permalink] New post 04 Mar 2012, 04:12
Bunuel wrote:
carcass wrote:
If there is exactly one root of the equation x^2 + ax + b, where a and b are positive constants, what is b in terms of a?

A. a/2
B. a
C. 3a/2
D. a^2/2
E. a^2/4

I'm not sure how to solve this problem. It take me almost five minutes of brainstorming but nothing.

The only thing on how I 'm triyng to attack the same is : (x+b)^2 whre the only root is x=-b.

:(


Yes, you can solve it this way: x^2+ax+b=0 will have only one root if it can be factored as (x+n)^2=0, in this case the root will be x=-n. (x+n)^2=x^2+2nx+n=0 --> a=2n and b=n^2. Now, since n=\frac{a}{2}, then b=(\frac{a}{2})^2=\frac{a^2}{4}.

Answer: E.

Or: a quadratic function is ax^2+bx+c=0 will have only one root (only one intercept with x-axis) if discriminant is zero, so when discriminant=b^2-4ac=0.

For given expression x^2+ax+b=0 discriminant is a^2-4b, so it must equal to zero: a^2-4b=0 --> b=\frac{a^2}{4}.

Answer: E.

Or: try number plugging, x^2+ax+b=0 will have only one root if it can be factored for example as (x+4)^2=0 --> x^2+8x+16=0 --> a=8 and b=16. Now, plug a=8 in the answer choices and see which one gives b=16, only answer choice E works.

Answer: E.

Note that for plug-in method it might happen that for some particular number(s) more than one option may give "correct" answer. In this case just pick some other numbers and check again these "correct" options only. For example if you pick (x+2)^2=0 then you get two "correct" options B and E.

Hope it helps.


OMG in red is the best part. Why I didn't think to the quadratic formula where if positive we have 2 solutions, = 0 ONE solution, < 0 NO solution. In less than 10 seconds .

Thanks Bunuel.
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Re: If there is exactly one root of the equation x^2 + ax + b   [#permalink] 04 Mar 2012, 04:12
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