Alternative Solution to a Quadratic Equation

Vertciel

Junior Member
Joined
May 13, 2007
Messages
78
Hello everyone,

I solved this quadratic equation using logic, but my math teacher said that it could also be solved using the Quadratic Formula. I do not how to do the latter because it seems that the discriminant would become negative (Imaginary numbers have not been taught yet to the class, so the question shouldn't involve them).

Any help would be appreciated!

Thank you.

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1. Solve the following for x in terms of y:

\(\displaystyle x^2 - 2x + 1 + y^2 = 0\)

My method :

\(\displaystyle (x - 1)^2 + y^2 = 0\)

\(\displaystyle x = 1\), because for the sum of any two positive numbers to be zero, the two positive numbers must be 0 themselves.

Teacher's method :

a = 1, b = -2, c = \(\displaystyle y^2 + 1\)

Therefore, the quadratic formula would yield:

\(\displaystyle x = \frac{2 \pm \sqrt{4 - 4(y^2 + 1)}}{2}\)

\(\displaystyle x = \frac{2 \pm \sqrt{4 - 4y^2 - 4}}{2}\)

\(\displaystyle x = 1 \pm \frac{ \sqrt{- 4y^2}}{2}\)
 
Well your teacher wants to solve for x in terms of y. It's true that you'll get an imaginary number with your discriminant IFF -4y<sup>2</sup> < 0.

As you deduced from your method, the two positive numbers ( (x-1)<sup>2</sup> and y<sup>2</sup>) must be zero. Keeping that in mind as we look at the expression you got using the quadratic formula, then you can see y = 0 is the only real solution that would work - also giving you x = 1.
 
I think your method is much more elegant and straight forward.

Look at the equation you got.

It is a circle - centered at (1,0) - with a radius = 0.

Thus it is the point (1,0) - giving you x=1 and y=0 (your conclusion - geometrically).
 
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