not to be republished

1 IntroductionYou have studied many properties of a triangle in Chapters 6 and 7 and you know thaton joining three non-collinear points in pairs, the figure so obtained is a triangle. Now,let us mark four points and see what we obtain on joining them in pairs in some that if all the points are collinear (in the same line), we obtain a linesegment [see Fig. (i)], if three out of four points are collinear, we get a triangle[see Fig. (ii)], and if no three points out of four are collinear, we obtain a closedfigure with four sides [see Fig. (iii) and (iv)].Such a figure formed by joining four points in an order is called a this book, we will consider only quadrilaterals of the type given in Fig. (iii) butnot as given in Fig. (iv).A quadrilateral has four sides, four angles and four vertices [see Fig. (i)].Fig. NCERTnot to be republished136 MATHEMATICSIn quadrilateral ABCD, AB, BC, CD and DA are the four sides; A, B, C and D arethe four vertices and A, B, C and D are the four angles formed at join the opposite vertices A to C and B to D [see Fig.]

2 (ii)].AC and BD are the two diagonals of the quadrilateral this chapter, we will study more about different types of quadrilaterals, theirproperties and especially those of may wonder why should we study about quadrilaterals (or parallelograms)Look around you and you will find so many objects which are of the shape of aquadrilateral - the floor, walls, ceiling, windows of your classroom, the blackboard,each face of the duster, each page of your book, the top of your study table etc. Someof these are given below (see Fig. ).Fig. most of the objects we see around are of the shape of special quadrilateralcalled rectangle, we shall study more about quadrilaterals and especially parallelogramsbecause a rectangle is also a parallelogram and all properties of a parallelogram aretrue for a rectangle as Angle Sum Property of a QuadrilateralLet us now recall the angle sum property of sum of the angles of a quadrilateral is 360.

3 This can be verified by drawing a diagonal and dividingthe quadrilateral into two ABCD be a quadrilateral and AC be adiagonal (see Fig. ).What is the sum of angles in ADC?Fig. NCERTnot to be republishedQUADRILATERALS137 You know that DAC + ACD + D = 180 (1)Similarly, in ABC, CAB + ACB + B = 180 (2)Adding (1) and (2), we get DAC + ACD + D + CAB + ACB + B = 180 + 180 = 360 Also, DAC + CAB = A and ACD + ACB = CSo, A + D + B + C = 360 . , the sum of the angles of a quadrilateral is 360 . Types of QuadrilateralsLook at the different quadrilaterals drawn below:Fig. that : One pair of opposite sides of quadrilateral ABCD in Fig. (i) namely, ABand CD are parallel. You know that it is called a trapezium. Both pairs of opposite sides of quadrilaterals given in Fig. (ii), (iii) , (iv)and (v) are parallel.

4 Recall that such quadrilaterals are called , quadrilateral PQRS of Fig. (ii) is a parallelogram. NCERTnot to be republished138 MATHEMATICSS imilarly, all quadrilaterals given in Fig. (iii), (iv) and (v) are parallelograms. In parallelogram MNRS of Fig. (iii), note that one of its angles namely M is a right angle. What is this special parallelogram called? Try to is called a rectangle. The parallelogram DEFG of Fig. (iv) has all sides equal and we know thatit is called a rhombus. The parallelogram ABCD of Fig. (v) has A = 90 and all sides equal; itis called a square. In quadrilateral ABCD of Fig. (vi), AD = CD and AB = CB , two pairsof adjacent sides are equal. It is not a parallelogram. It is called a that a square, rectangle and rhombus are all parallelograms. A square is a rectangle and also a rhombus. A parallelogram is a trapezium. A kite is not a parallelogram.

5 A trapezium is not a parallelogram (as only one pair of opposite sides is parallelin a trapezium and we require both pairs to be parallel in a parallelogram). A rectangle or a rhombus is not a at the Fig. We have a rectangle and a parallelogram with same perimeter14 the area of the parallelogram is DP AB and this is less than the area of therectangle, , AB AD as DP < AD. Generally sweet shopkeepers cut Burfis in theshape of a parallelogram to accomodate more pieces in the same tray (see the shapeof the Burfi before you eat it next time!).Let us now review some properties of a parallelogram learnt in earlier classes. NCERTnot to be Properties of a ParallelogramLet us perform an out a parallelogram from a sheet of paperand cut it along a diagonal (see Fig. ). You obtaintwo triangles. What can you say about thesetriangles?Place one triangle over the other.

6 Turn one around,if necessary. What do you observe?Observe that the two triangles are congruent toeach this activity with some more parallelograms. Each time you will observethat each diagonal divides the parallelogram into two congruent us now prove this : A diagonal of a parallelogram divides it into two : Let ABCD be a parallelogram and AC be a diagonal (see Fig. ). Observethat the diagonal AC divides parallelogram ABCD into two triangles, namely, ABCand CDA. We need to prove that these triangles are ABC and CDA, note that BC || AD and AC is a , BCA = DAC (Pair of alternate angles)Also,AB || DC and AC is a , BAC = DCA (Pair of alternate angles)andAC = CA(Common)So, ABC CDA(ASA rule)or,diagonal AC divides parallelogram ABCD into two congruenttriangles ABC and CDA. Now, measure the opposite sides of parallelogram ABCD.

7 What do you observe?You will find that AB = DC and AD = is another property of a parallelogram stated below:Theorem : In a parallelogram, opposite sides are have already proved that a diagonal divides the parallelogram into two congruentFig. NCERTnot to be republished140 MATHEMATICS triangles; so what can you say about the corresponding parts say, the correspondingsides? They are ,AB = DC and AD = BCNow what is the converse of this result? You already know that whatever is givenin a theorem, the same is to be proved in the converse and whatever is proved in thetheorem it is given in the converse. Thus, Theorem can be stated as given below :If a quadrilateral is a parallelogram, then each pair of its opposite sides is equal. Soits converse is :Theorem : If each pair of opposite sides of a quadrilateral is equal, then itis a you reason out why?

8 Let sides AB and CD of the quadrilateral ABCDbe equal and also AD = BC (see Fig. ). Drawdiagonal , ABC CDA(Why?)So, BAC = DCAand BCA = DAC(Why?)Can you now say that ABCD is a parallelogram? Why?You have just seen that in a parallelogram each pair of opposite sides is equal andconversely if each pair of opposite sides of a quadrilateral is equal, then it is aparallelogram. Can we conclude the same result for the pairs of opposite angles?Draw a parallelogram and measure its angles. What do you observe?Each pair of opposite angles is this with some more parallelograms. We arrive at yet another result asgiven : In a parallelogram, opposite angles are , is the converse of this result also true? Yes. Using the angle sum property ofa quadrilateral and the results of parallel lines intersected by a transversal, we can seethat the converse is also true.

9 So, we have the following theorem :Theorem : If in a quadrilateral, each pair of opposite angles is equal, thenit is a NCERTnot to be republishedQUADRILATERALS141 There is yet another property of a parallelogram. Let us study the same. Draw aparallelogram ABCD and draw both its diagonals intersecting at the point O(see Fig. ).Measure the lengths of OA, OB, OC and do you observe? You will observe thatOA = OC and OB = ,O is the mid-point of both the this activity with some more time you will find that O is the mid-point of both the , we have the following theorem :Theorem : The diagonals of a parallelogrambisect each , what would happen, if in a quadrilateralthe diagonals bisect each other? Will it be aparallelogram? Indeed this is result is the converse of the result ofTheorem It is given below:Theorem : If the diagonals of a quadrilateralbisect each other, then it is a can reason out this result as follows:Note that in Fig.

10 , it is given that OA = OCand OB = , AOB COD(Why?)Therefore, ABO = CDO(Why?)From this, we get AB || CDSimilarly, BC || ADTherefore ABCD is a us now take some 1 : Show that each angle of a rectangle is a right : Let us recall what a rectangle rectangle is a parallelogram in which one angle is a right NCERTnot to be republished142 MATHEMATICS Let ABCD be a rectangle in which A = 90 .We have to show that B = C = D = 90 We have, AD || BC and AB is a transversal(see Fig. ).So, A + B = 180 (Interior angles on the same side of the transversal)But, A = 90 So, B = 180 A = 180 90 = 90 Now, C = A and D = B(Opposite angles of the parallellogram)So, C = 90 and D = 90 .Therefore, each of the angles of a rectangle is a right 2 : Show that the diagonals of a rhombus are perpendicular to each : Consider the rhombus ABCD (see Fig.)