Explanation:
a)Area of ABD=Area of ACB
Area of ABCD=2×Area of ABD
provided that Area of ABD =Area of ACB
Area of ABCD=Area of ABD+Area of BCD
A=2×20
Area of ABCD=40cm²
L×B=40
b)The two triangles in the diagram are
Triangle DOC and AOB
where O is the centre