y=π* 18^2-π*x^2

That is y =-π x 2-324π.

(2) When the radius of the small circle changes from 1 cm to 9 cm, the area of the circular ring changes from

323π cubic centimeters decreased to 243π cubic centimeters.

20.？

I hope I can help you and adopt it. I wish you progress in your study.