Answer:
Radius: [tex]r =\frac{\sqrt {21}}{6}[/tex]
[tex]Center = (-\frac{3}{2}, -\frac{2}{3})[/tex]
Step-by-step explanation:
Given
[tex]9x^2 + 9y^2 + 27x + 12y + 19 = 0[/tex]
Solving (a): The radius of the circle
First, we express the equation as:
[tex](x - h)^2 + (y - k)^2 = r^2[/tex]
Where
[tex]r = radius[/tex]
[tex](h,k) =center[/tex]
So, we have:
[tex]9x^2 + 9y^2 + 27x + 12y + 19 = 0[/tex]
Divide through by 9
[tex]x^2 + y^2 + 3x + \frac{12}{9}y + \frac{19}{9} = 0[/tex]
Rewrite as:
[tex]x^2 + 3x + y^2+ \frac{12}{9}y =- \frac{19}{9}[/tex]
Group the expression into 2
[tex][x^2 + 3x] + [y^2+ \frac{12}{9}y] =- \frac{19}{9}[/tex]
[tex][x^2 + 3x] + [y^2+ \frac{4}{3}y] =- \frac{19}{9}[/tex]
Next, we complete the square on each group.
For [tex][x^2 + 3x][/tex]
1: Divide the [tex]coefficient\ of\ x\ by\ 2[/tex]
2: Take the [tex]square\ of\ the\ division[/tex]
3: Add this [tex]square\ to\ both\ sides\ of\ the\ equation.[/tex]
So, we have:
[tex][x^2 + 3x] + [y^2+ \frac{4}{3}y] =- \frac{19}{9}[/tex]
[tex][x^2 + 3x + (\frac{3}{2})^2] + [y^2+ \frac{4}{3}y] =- \frac{19}{9}+ (\frac{3}{2})^2[/tex]
Factorize
[tex][x + \frac{3}{2}]^2+ [y^2+ \frac{4}{3}y] =- \frac{19}{9}+ (\frac{3}{2})^2[/tex]
Apply the same to y
[tex][x + \frac{3}{2}]^2+ [y^2+ \frac{4}{3}y +(\frac{4}{6})^2 ] =- \frac{19}{9}+ (\frac{3}{2})^2 +(\frac{4}{6})^2[/tex]
[tex][x + \frac{3}{2}]^2+ [y +\frac{4}{6}]^2 =- \frac{19}{9}+ (\frac{3}{2})^2 +(\frac{4}{6})^2[/tex]
[tex][x + \frac{3}{2}]^2+ [y +\frac{4}{6}]^2 =- \frac{19}{9}+ \frac{9}{4} +\frac{16}{36}[/tex]
Add the fractions
[tex][x + \frac{3}{2}]^2+ [y +\frac{4}{6}]^2 =\frac{-19 * 4 + 9 * 9 + 16 * 1}{36}[/tex]
[tex][x + \frac{3}{2}]^2+ [y +\frac{4}{6}]^2 =\frac{21}{36}[/tex]
[tex][x + \frac{3}{2}]^2+ [y +\frac{4}{6}]^2 =\frac{7}{12}[/tex]
[tex][x + \frac{3}{2}]^2+ [y +\frac{2}{3}]^2 =\frac{7}{12}[/tex]
Recall that:
[tex](x - h)^2 + (y - k)^2 = r^2[/tex]
By comparison:
[tex]r^2 =\frac{7}{12}[/tex]
Take square roots of both sides
[tex]r =\sqrt{\frac{7}{12}}[/tex]
Split
[tex]r =\frac{\sqrt 7}{\sqrt 12}[/tex]
Rationalize
[tex]r =\frac{\sqrt 7*\sqrt 12}{\sqrt 12*\sqrt 12}[/tex]
[tex]r =\frac{\sqrt {84}}{12}[/tex]
[tex]r =\frac{\sqrt {4*21}}{12}[/tex]
[tex]r =\frac{2\sqrt {21}}{12}[/tex]
[tex]r =\frac{\sqrt {21}}{6}[/tex]
Solving (b): The center
Recall that:
[tex](x - h)^2 + (y - k)^2 = r^2[/tex]
Where
[tex]r = radius[/tex]
[tex](h,k) =center[/tex]
From:
[tex][x + \frac{3}{2}]^2+ [y +\frac{2}{3}]^2 =\frac{7}{12}[/tex]
[tex]-h = \frac{3}{2}[/tex] and [tex]-k = \frac{2}{3}[/tex]
Solve for h and k
[tex]h = -\frac{3}{2}[/tex] and [tex]k = -\frac{2}{3}[/tex]
Hence, the center is:
[tex]Center = (-\frac{3}{2}, -\frac{2}{3})[/tex]
