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LanceVance's avatar

What is x, a and b of the following problem?

Asked by LanceVance (645points) September 29th, 2009

For which a and b are the solutions of the equation -4bx^2 + (a-2)x +a +b = 0 imaginary. (a,b are real numbers)

I have come to the point where I evaluated a to be 2, but am stuck with evaluating b.

Here’s the a process:

x² – ( (a-2) / 4b )x – ( (a+b) / 4b) = 0 //divided by -4b

x² – ( (a-2) / 4b )x + ((a-2)² / 64b²) = ((a+b)/4b) + ((a-2)² / 64b²) //completing the square

(x – (a-2)/8b)² = (16b(a+b) + (a-2)²) / 64b²

x = ±root(16b(a+b)+(a-2)²) / 8b + (a-2) / 8b =
__________________________________
from this follows that a-2 need be 0 (or a = 2) because the real part of the complex solution needs be zero (remember, we’re looking for a solution only with imaginary part).

any suggestion as to how to proceed with solving for b?

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6 Answers

Response moderated
fireinthepriory's avatar

Wow, I definitely don’t remember how to do this anymore… does your school have tutors you can ask? That might be a better bet than futher… We don’t really take well to answering homework questions…

LostInParadise's avatar

You know the portion in the squre root parentheses must be less than zero and you know a=2. Substitute 2 for a in the square root experssion

LanceVance's avatar

LostInParadise, i’ve tried doing as you said, but that way I get an inequality but the solution is only one number.

Sarcasm's avatar

Wolfram Alpha knows all the answers.

LostInParadise's avatar

@LanceVance , I do not see how b can only be one number. The way that the question is stated suggests that there is only one value of b, but it does not actually come out and say so. I would go on the assumption that a range of b values is acceptable. Once you get the range of b values, test your solution by trying a=2 and b equal to two different values in your range. See if you get an imaginary value in both cases.

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