The diagram shows a triangle DEF inside a rectangle ABCD.
Show that that the area of triangle DEF is 11 cm^2.
CD = sqrt((2√10)2 - 22) = 6 cm
[ABCD] = CD * AB = 30 cm2
[DEF] = [ABCD] - {1/2(4*5) + 1/2(2*3) + 1/2(2*6)} = 11 cm2