Question

what is an orthocentre,centroid and circumcircle

Answer 1

The centroid is the point of intersection of the medians of a triangle. A medium from a point, say A in the triangle ABC, is the line AD such that D is the midpoint of BC.

The orthocentre is the point of intersection of the perpendiculars of the triangle. A perpendicular is just that - the line AD such that AD is perpendicular to the side BC (opposite A).

circumcircle. -- a circle touching all the vertices of a triangle or polygon.