# Finding standard equation of an Ellipse

• April 27th 2013, 01:17 PM
vaironxxrd
Finding standard equation of an Ellipse
Hello Everyone,

I can't figure out very well how to find the standard equation of the following ellipse.

$25x^2+49y^2-300x+49y-1251/4=0$

I was trying the following steps.

$[25x^2-300x +- ?]+ [ 49y^2+49y +- ? ]= 1251/4$ Find squares (I'm not good at, finding squares)
• April 27th 2013, 01:28 PM
Shakarri
Re: Finding standard equation of an Ellipse
Looking at the x part. If you want to get it into the form (ax+b)2 expanding it out might make it clearer. (ax+b)2=a2x2+2abx+b2
since the x2 part is 25x2 you know that a2=25, a=5
The x part is -300. You know that 2ab=-300. and a=5 so b=-30
Lastly you need to balance the square by subtracting a constant.

$25x^2-300x=(5x-30)^2+c$
For this to be true c must be equal to -900
• April 27th 2013, 02:15 PM
Soroban
Re: Finding standard equation of an Ellipse
Hello, vaironxxrd!

Before completing-the-square,

Quote:

Find the standard equation of the following ellipse:

. . $25x^2+49y^2-300x+49y-\tfrac{1251}{4}\:=\:0$

We have: . . $25x^2 - 300x \quad+ 49y^2 + 49y\quad \:=\:\tfrac{1251}{4}$

Factor:. $25(x^2-12x \qquad) + 49(y^2 + y \qquad) \:=\:\tfrac{1251}{4}$

. . . . . . $25(x^2-12x + {\color{red}36}) + 49(y^2 + y +{\color{blue}\tfrac{1}{4}}) \:=\:\tfrac{1251}{4} + {\color{red}900} + {\color{blue}\tfrac{49}{4}}$

. . . . . . . . . . . . . . $25(x-6)^2 + 49(y+\tfrac{1}{2})^2 \:=\:1225$

Divide by 1225:. . . . . $\frac{(x-6)^2}{49} + \frac{(y+\frac{1}{2})^2}{25} \;=\;1$