How do I convert the equation x^2+y^2=9 into polar coordinates?

Answer 1

 x= r cos(θ) 

 y=r sin(θ)

x²+y²=9

r²cos²(θ)+r²sin²(θ)=9

r²(cos²(θ)+sin²(θ))=9

 cos²(θ)+sin²(θ)=1

 r² (1) =9 

 r=3

Answer 2

x = rcos(θ)

y = rsin(θ)

r²cos²(θ) + r²sin²(θ) = 9