([i]Drag the three vertices of the triangle to change its shape.[/i])[br][br]Drag the blue point to A.[br]Drag the centre (the red point) to different positions at which the circle also passes through B.[br]Observe the marks of these positions. What are their characteristic?[br][br]What is the charateristic of the positions of the centre at which the circle passes through A and C?[br][br]How to draw the circle which passes thorough A, B and C?

Anthony Or. GeoGebra Institute of Hong Kong.