Demonstration on area of a circle.
https://www.geogebra.org/m/hS2XZbRk
https://www.geogebra.org/m/AJSmWRzp
https://www.geogebra.org/m/DGyTYERs#material/awBAYg6P
Demonstration on circumference of a circle
https://www.geogebra.org/m/wZS2aECm