Answer:
The radius is 10
Step-by-step explanation:
Given
[tex]x^2 + y^2 + 21 = 40 + 18y.[/tex]
Required
The radius
Rewrite as:
[tex]x^2 + y^2 - 18y = 40-21[/tex]
Subtract 81 from both sides
[tex]x^2 + y^2 - 18y +81= 40-21+81[/tex]
Expand
[tex]x^2 + y^2 - 9y - 9y +81= 40-21+81[/tex]
Factorize
[tex]x^2 + y( y- 9) - 9(y -9)= 40-21+81[/tex]
Factor out y - 9
[tex]x^2 + (y- 9) (y -9)= 40-21+81[/tex]
Express as squares
[tex]x^2 + (y- 9)^2= 100[/tex]
[tex]x^2 + (y- 9)= 10^2[/tex]
The equation of a circle is:
[tex](x - a)^2 + (y- b)= r^2[/tex]
By comparison:
[tex]r^2=10^2[/tex]
[tex]r = 10[/tex]
