NCERT Solutions
Class 8 Maths
Understanding Quadrilaterals

Ex. 3.1 Q3.
What is the sum of the measures of the angles of a convex quadrilateral? Will this property hold if the quadrilateral is not convex? (Make a non-convex quadrilateral and try)
Let ABCD is a convex quadrilateral, then we draw a diagonal AC which divides the quadrilateral in two triangles.
Ð A + ÐB + Ð C + Ð D = Ð 1 + Ð 6 + Ð 5 + Ð 4 + Ð 3 + Ð 2
= (Ð 1 + Ð 2 + Ð 3) + (Ð 4 + Ð 5 + Ð 6)
= 1800 + 1800 [By Angle sum property of triangle]
= 3600
Hence, the sum of measures of the triangles of a convex quadrilateral is 3600.
Yes, if quadrilateral is not convex then, this property will also be applied.
Let ABCD is a non-convex quadrilateral and join BD, which also divides the quadrilateral in two triangles.
Using angle sum property of triangle,
In Δ ABD, Ð 1 + Ð 2 + Ð 3 = 1800 …………….1
In Δ BDC, Ð 4 + Ð 5 + Ð 6 = 1800 …………….2
Adding equation 1 and 2, we get
Ð 1 + Ð 2 + Ð 3 + Ð 4 + Ð 5 + Ð 6 = 1800 + 1800
=> Ð 1 + Ð 2 + Ð 3 + Ð 4 + Ð 5 + Ð 6 = 3600
=> Ð A + ÐB + Ð C + Ð D = 3600
Hence proved.