# Thread: Solving for x and y

1. ## Solving for x and y

solve &#91;&#40;x&#94;2&#41;&#47;25 &#43; &#40;y&#94;2&#41;&#47;4 -25&#61;0, &#40;x&#94;2&#41;&#47;16 &#43; &#40;y&#94;2&#41;&#47;9 -29&#61;0, x, y&#93; - Wolfram|Alpha

I am not certain what the method is. I have made and equation y^2=... from the first equation and put it into the other but I did not get +or-20.

2. ## Re: Solving for x and y

$\frac{x^2}{25}+\frac{y^2}{4}=25$

Multiply through by $4$:

$\frac{4x^2}{25}+y^2=100$

$y^2=100-\frac{4x^2}{25}$

What did you get when you substituted this in and tried to solve?

3. ## Re: Solving for x and y

X^2=1299600/2601 X=+/- 22.35 x is +/-20.

4. ## Re: Solving for x and y

I'm fairly sure that might be overcomplicating things.

$y^2=100-\frac{4x^2}{25}$

Substituting this into $\frac{x^2}{16}+\frac{y^2}{9}-29=0$ gives:

$\frac{x^2}{16}+\frac{100-\frac{4x^2}{25}}{9}-29=0$

Multiplying by $9:$

$\frac{9}{16}x^2+100-\frac{4}{25}x^2-261=0$

$\frac{161}{400}x^2=161$

Does that clarify?

5. ## Re: Solving for x and y

I have typed the hyperbola correctly by accident. The book wrote 21 not 29 and I found that error. Also it had a negative sign instead of positive.

6. ## Re: Solving for x and y

The joy of serendipity.

7. ## Re: Solving for x and y

Another way:

x^2/25 + y^2/4 = 25 [1]
x^2/16 + y^2/9 = 29 [2]

[1] *100: 4x^2 + 25y^2 = 2500 [1]
[2] * 144: 9x^2 + 16y^2 = 4176 [1]

36x^2 + 225y^2 = 22500 : [1]*9
-36x^2 - 64y^2 = -16704 : [2]*-9