1. what is a hollow circle ?
2. what is a perfect Circle ?
3.Heron's formula

2. the collection of all points in a plane that are the same distance from a central point is called a perfect circle.

1) The hollow circle is one which is empty from inside

2) perfect circle has infinite sides and points which are at equal distance from the centre

3) Heron's formula = Root S ( s- a) (s - b) ( s - c)

step-by-step explanation:

no, her answer is not correct.   we square the lengths of all sides and then determine whether the pythagorean theorem holds here:

9² + 15² = 306.   this is not equal to the square of the longest side ( that square being 395.   answer c is correct.