We know that
the equation of a circle is
(x-h)²+(y-k)²=r²
where
(h,k)-----> is the center
r--------> is the radius
is the center is the point (0,0) and radius r=20
then
(h,k) is (0,0)
(x-0)²+(y-0)²=20²---------> (x)²+(y)²=400
the answer is
the option D.) x2 + y2 = 400
Answer:
![\sqrt[]{\frac{x+8}{4}}-3](https://tex.z-dn.net/?f=%5Csqrt%5B%5D%7B%5Cfrac%7Bx%2B8%7D%7B4%7D%7D-3)
Step-by-step explanation:

First rewrite
as y

Now swap y and x

Add 8 on both sides.


Divide by 4.


Extract the square root on both sides.
![\sqrt[]{\frac{x+8}{4}}=\sqrt[]{(y+3)^2}](https://tex.z-dn.net/?f=%5Csqrt%5B%5D%7B%5Cfrac%7Bx%2B8%7D%7B4%7D%7D%3D%5Csqrt%5B%5D%7B%28y%2B3%29%5E2%7D)
![\sqrt[]{\frac{x+8}{4}}=y+3](https://tex.z-dn.net/?f=%5Csqrt%5B%5D%7B%5Cfrac%7Bx%2B8%7D%7B4%7D%7D%3Dy%2B3)
Subtract 3 on both sides.
![\sqrt[]{\frac{x+8}{4}}-3=y+3-3](https://tex.z-dn.net/?f=%5Csqrt%5B%5D%7B%5Cfrac%7Bx%2B8%7D%7B4%7D%7D-3%3Dy%2B3-3)
![\sqrt[]{\frac{x+8}{4}}-3=y](https://tex.z-dn.net/?f=%5Csqrt%5B%5D%7B%5Cfrac%7Bx%2B8%7D%7B4%7D%7D-3%3Dy)
The average speed = (speed 1 + speed 2)/2
the average speed = (80/10 + 100/20)/2
the average speed = (8 + 5)/2
the average speed = 13/2
the average speed = 6.5 km/minute