If two triangles have equal area and stand on the same base, then the altitudes of triangle are equal.
1: AB-b; 2: AD-a; 3: BC-c
Triangles on the same base (or equal bases) and having equal areas have equal altitudes.
This can be easily proved. Take two triangles: triangle-1 and triangle-2 having same base 'b' and altitudes 'c' and 'd' respectively.
Area of Triangle-1 = 12×b×c
Area of Triangle-2 = 12×b×d
Since, the triangles have same area, c = d