In the given applet AB is parallel to FD.
Use slider AB to change length of common base AB and slider BD to change length of the other side of parallelogram, see the corresponding change in the area of traingle and parallelogram. What do you observe? Does they have any relation?
NOTE: You can also drag points A, B and H.
C.I.E.T, NCERT, New Delhi, 10 December 2013, Created with GeoGebra