Theory explained in text

A median of a triangle is the line segment that joins (runs from) any vertex of the triangle with the mid-point of its opposite side. In the figure shown below, the median from A meets the mid-point of the opposite side, BC, at point D. Make sure you mark the 2 halfs of the side with the same sign.

The properties of the medians: It cuts the triangle into smaller triangles that have the same area's (oppervlakte). It also cuts the side of the triangle into 2 line segments that have the same sides.

The centroid of a triangle is the intersection of the three medians of the triangle (each median connecting a vertex with the midpoint of the opposite side).

 

An altitude is a line which passes through a vertex of the triangle and is perpendicular to the opposite side. There are therefore three altitudes in a triangle. Make sure you mark the perpendicular line with the sign of a right angle

An altitude of a triangle can be a side or may lie outside the triangle.
The orthocenter is the point where all three altitudes of the triangle intersect.
Altitude (triangle) - Wikipedia