# PROPERTIES OF TRIANGLES AND QUADRILATERALS

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1 Mathematics Revision Guides Properties of Triangles, Quadrilaterals and Polygons Page 1 of 21 M.K. HOME TUITION Mathematics Revision Guides Level: GCSE Higher Tier PROPERTIES OF TRIANGLES AND QUADRILATERALS (plus polygons in general) Version: 2.32 Date:

2 Mathematics Revision Guides Properties of Triangles, Quadrilaterals and Polygons Page 2 of 21 PROPERTIES OF TRIANGLES AND QUADRILATERALS Revision. Here is a recap of the methods used for calculating areas of quadrilaterals and triangles. There are more examples in the Foundation Tier document Measuring Shapes.

3 Mathematics Revision Guides Properties of Triangles, Quadrilaterals and Polygons Page 3 of 21 Types of triangles. Triangles can be classified in various ways, based either on their symmetry or their angle properties. An equilateral triangle has all three sides equal in length, and all three angles equal to 60. An isosceles triangle has two sides equal, and the two angles opposite the equal sides also the same. Thus in the diagram, angle A = angle C. A scalene triangle has all three sides and angles different. All triangles have an interior angle sum of 180. A scalene triangle has no symmetry at all, but an isosceles triangle has one line of symmetry. An equilateral triangle has three lines of symmetry, and rotational symmetry of order 3 in addition. Another way of classifying triangles is by the types of angle they contain. An acute-angled triangle has all its angles less than 90. A right-angled triangle has one 90 angle, and an obtuse-angled triangle has one obtuse angle. Because the sum of the internal angles of any triangle is 180, it follows that no triangle can have more than one right angle or obtuse angle.

4 Mathematics Revision Guides Properties of Triangles, Quadrilaterals and Polygons Page 4 of 21 Example (1): i) Find the angles A, B and C in the triangles below. ii) Is it possible for a right-angled triangle to be isosceles? Explain with a sketch. i) In the first triangle, the given angle is the different one, so therefore the two equal angles must add to (180-46), or 134. Angle A = half of 134, or 67. In the second triangle, the 66 angle is one of the two equal ones. Hence angle B = ( ), or 48. The third triangle is scalene, so angle C = ( ) = 47. ii) Although no triangle can have two right angles, it is perfectly possible to have an isosceles rightangled triangle. Two such triangles can be joined at their longest sides to form a square, and this shape of triangle occurs in a set square of a standard geometry set.

5 Mathematics Revision Guides Properties of Triangles, Quadrilaterals and Polygons Page 5 of 21 Here is the proof that the interior angles of any triangle add up to 180. The exterior angle sum of a triangle. Since an exterior angle is obtained by extending one of the sides, it follows that the sum of an interior angle and the resulting exterior angle is 180. See the diagram. The interior angle-sum, A + B + C, = 180. The exterior angle sum is ( (180 - A) + (180 B) + (180 C)) or (540 (A + B + C)) or = 360. The exterior angle sum of a triangle is 360.

6 Mathematics Revision Guides Properties of Triangles, Quadrilaterals and Polygons Page 6 of 21 Congruent Triangles. Two triangles are said to be congruent if they are equal in size and shape. In other words, if triangle B can fit directly onto triangle A by any combination of the three standard transformations (translation, rotation, reflection), then triangles A and B are congruent. Conditions for congruency. 3 sides (SSS) If three sides of one triangle are equal to the three sides of the other, then the triangles are congruent. (Note that the triangles are mirror images of each other, but they are still congruent.) 2 sides and the included angle (SAS) If two sides and the included angle of one triangle are equal to two sides and the included angle of the other, then the triangles are congruent.

7 Mathematics Revision Guides Properties of Triangles, Quadrilaterals and Polygons Page 7 of 21 2 angles and a corresponding side (AAS) If two angles and a corresponding side of one triangle are equal to two angles and a corresponding side of the other, then the triangles are congruent. Here, the sides correspond, as each one is opposite the unmarked angle (here 81 by subtraction). Of course, if two angles of each triangle are equal, then the third angle must also be equal. (Right-angled triangles) Hypotenuse and one side (RHS) If the hypotenuse and one side of one right-angled triangle are equal to the hypotenuse and one side of the other, then the triangles are congruent.

8 Mathematics Revision Guides Properties of Triangles, Quadrilaterals and Polygons Page 8 of 21 Conditions insufficient for congruency. The four conditions above guarantee congruency of triangles, but the following do not: 2 angles and a non-corresponding side The two triangles above are not congruent because the 60mm sides do not correspond. In the left-hand triangle, the 60mm side is opposite the 81 angle, but in the right-hand triangle, the 60mm side is opposite the 57 angle. The triangles do have the same shape, and are therefore similar. 3 angles This is a generalisation of the above case - the sides opposite the 81 angle are not equal. The triangles have the same shape, therefore they are similar, but not congruent.

9 Mathematics Revision Guides Properties of Triangles, Quadrilaterals and Polygons Page 9 of 21 An angle not included and 2 sides The triangles above have two sides equal at 45mm and 65mm, but the angle of 36 is crucially not the included angle. They are emphatically not congruent - in fact they are not even similar. The reason is as follows: when attempting to construct the 45mm side, the compass arc centred on O cuts the third side either at B or C, giving two possible constructs namely triangles OAB and OAC. Example (2a): ABCD is a kite. (See the section on quadrilaterals later in the document.) Prove that the diagonal AC divides the kite into two congruent triangles ABC and ADC. A kite has adjacent pairs of sides equal, so AB = AD and BC = CD. Also, the diagonal of this kite, AC, is common to both triangles. Hence three sides of triangle ABC = three sides of triangle ADC. triangles ABC and ADC are congruent (SSS).

10 Mathematics Revision Guides Properties of Triangles, Quadrilaterals and Polygons Page 10 of 21 Example (2b): The illustrated figure is made up of a square and two equilateral triangles, each of which has a side length of one unit. Prove that triangles CDE and BCF are congruent. The interior angle of a square is 90 and the interior angle of an equilateral triangle is 60. EDC = BCF = = 150. Also, BC = CF = 1 unit and CD = DE = 1 unit. Hence two sides and the included angle of triangle CDE are equal to two sides and the included angle of triangle BCF. triangles CDE and BCF are congruent (SAS). Example (2c) : Figures ABCD and AEFG are squares. Prove, using congruent triangles, that DG = EB. The triangles in question are AEB and AGD. Now, AG = AE and AD = AB, so two sides of one triangle are equal to two sides of the other. Let the included angle BAE of triangle AEB = x. Because BAE + BAG = EAG = 90, BAG = (90-x). Also, BAD = 90, so DAG = 90 - BAG, or 90 - (90 x) = x. the included angles BAE and DAG are both equal to x. Two sides and the included angle of triangle AEB are equal to two sides and the included angle of triangle AEG. triangles AEB and AEG are congruent (SAS).

12 Mathematics Revision Guides Properties of Triangles, Quadrilaterals and Polygons Page 12 of 21 Example (3): Find angle C in this quadrilateral, given that angle A = 70, angle B = 96 and angle D = angle A. C = 360 ( ) = ( ) = 124. We shall now start to examine the properties of some more special quadrilaterals. The trapezium. If one pair of sides is parallel, the quadrilateral is a trapezium. A trapezium whose non-parallel sides are equal in length is an isosceles trapezium. In the diagram, sides BC and AD are equal in length. An isosceles trapezium also has adjacent pairs of angles equal. Thus, angles A and B are equal, as are angles C and D. Furthermore, the diagonals of an isosceles trapezium are also equal in length, thus AC = BD. There is also a line of symmetry passing through the midpoints P and Q of the parallel sides as well as point M, the intersection of the diagonals. (Although, in this example, the diagonals appear to meet at right angles, this is not generally true.)

13 Mathematics Revision Guides Properties of Triangles, Quadrilaterals and Polygons Page 13 of 21 The kite. A kite has one pair of opposite angles and both pairs of adjacent sides equal. In the diagram, sides AB and AD are equal in length, as are CB and CD. Also, angles B and D are equal. A kite is symmetrical about one diagonal, which bisects it into two mirror-image congruent triangles. In this case, the diagonal AC is the line of symmetry, and triangles ABC and ADC are thus congruent. Hence AC also bisects the angles BAD and BCD, BX = XD, and the diagonals AC and BD intersect at right angles.

14 Mathematics Revision Guides Properties of Triangles, Quadrilaterals and Polygons Page 14 of 21 The parallelogram. Whereas a trapezium has at one pair of sides parallel, a parallelogram has both pairs of opposite sides equal and parallel, as well as having opposite pairs of angles equal. Thus, in the diagram, side AD = side BC, and side AB = side CD. Additionally, angle A = angle C, and angle B = angle D. The diagonals of a parallelogram bisect each other, so in the diagram below, AM = MC and BM = MD. We also have rotational symmetry of order 2 about the point M, the midpoint of each diagonal. Note however that a parallelogram does not generally have a line of symmetry.

15 Mathematics Revision Guides Properties of Triangles, Quadrilaterals and Polygons Page 15 of 21 The rhombus. A rhombus has all the properties of a parallelogram, but it also has all its sides equal in length. Hence here, AB = BC = CD = DA. Although a parallelogram does not as a rule have any lines of symmetry, a rhombus has two lines of symmetry coinciding with the diagonals, as well as order-2 rotational symmetry. Thus, triangles ABC and ACD are congruent, as are triangles BAD and BCD. The diagonals bisect each other at right angles, so AM = MC and BM = MD. The diagonals also bisect the angles of the rhombus.

16 Mathematics Revision Guides Properties of Triangles, Quadrilaterals and Polygons Page 16 of 21 The rectangle. A rectangle also has all the properties of a parallelogram, but it additionally has all its four angles equal to 90. Hence angles A, B, C and D are all right angles. As stated earlier, a parallelogram does not generally have any lines of symmetry, but a rectangle has two lines of symmetry passing through lines joining the midpoints of opposite pairs of sides, i.e. PQ and RS in the diagram. A rectangle also has order-2 rotational symmetry. The diagonals of a rectangle are equal in length and bisect each other, therefore AC = BD, and AM = MC = BM = MD. They do not generally bisect at right angles, though. Thus, triangles AMB and CMD are congruent, as are triangles BMC and AMD. It can be seen that both the rectangle and the rhombus are not quite perfect when it comes to symmetry, although they complement each other. The rhombus has all its sides equal, but not all its angles; the rectangle has all its angles equal to 90, but not all its sides. The diagonals of a rhombus meet at right angles, but are not equal in length, whereas those of a rectangle are equal in length but do not meet at right angles. That leaves us with one type of quadrilateral combining the best of both the rectangle and the rhombus.

17 Mathematics Revision Guides Properties of Triangles, Quadrilaterals and Polygons Page 17 of 21 The square. A square is the most perfect of quadrilaterals, combining the symmetries and properties of the rectangle and the rhombus. All four sides are equal. Thus AB = BC = CD = DA. All four angles A, B, C, D are right angles. Both pairs of sides are parallel. A square also has greater symmetry than a rectangle or a rhombus. There are now four lines of symmetry; two passing through the diagonals AC and BD, and the other two passing through the midpoints of opposite sides at PQ and RS. A square also has rotational symmetry of order 4. The diagonals bisect each other and are equal in length. Hence AC = BD, and AM = MB = CM = MD. They also bisect their respective angles - AC bisects angles A and C, and BD bisects angles B and D.

18 Mathematics Revision Guides Properties of Triangles, Quadrilaterals and Polygons Page 18 of 21 Polygons. A polygon is any plane figure with three or more straight sides. A regular polygon has all of its sides and angles equal. A few examples are shown as follows: Since the exterior angles of any convex polygon sum to 360, it follows that the exterior angle of a regular polygon (in degrees) is the number of sides divided into 360. Thus, for example, the exterior angle of a regular pentagon is 360 or 72, and its interior angle is 5 (180 72) or 108. Similarly, a regular hexagon has an exterior angle of 360 or 60, and its interior angle is Example (4): A regular polygon has all its interior angles equal to 140. How many sides does it have? If the interior angles of the polygon are all equal to 140, then its exterior angles must all be equal to ( ), or 40. Since all the exterior angles of the polygon sum to 360, it 360 follows that the number of sides is equal to 40 or 9. (The name for such a polygon is a regular nonagon).

19 Mathematics Revision Guides Properties of Triangles, Quadrilaterals and Polygons Page 19 of 21 Example (5): A floor is tiled using regular polygonal tiles. The diagram on the right shows part of the pattern. Two of the three types of polygon used are squares and regular hexagons. Calculate the number of sides in the third type of polygon. Three polygons meet at each corner of the pattern, namely a square, a regular hexagon and the unknown polygon. The interior angle of the square is 90, and the exterior angle of the hexagon is 360 or 60, so its interior angle is Since the angles around each corner add to 360, the third polygon has an interior angle of ( ), or 150. Hence one exterior angle of the third polygon is ( ) or 30 The number of sides in the third polygon is or 12, i.e. it is a dodecagon.

20 Mathematics Revision Guides Properties of Triangles, Quadrilaterals and Polygons Page 20 of 21 We have seen earlier how an equilateral triangle has 3 lines of symmetry and rotational symmetry of order 3; also, how a square has 4 lines of symmetry and rotational symmetry of order 4. This same pattern occurs in all regular polygons they have as many lines of symmetry as they have sides; likewise, their order of rotational symmetry is equal to the number of sides. See the example of a regular hexagon below.

21 Mathematics Revision Guides Properties of Triangles, Quadrilaterals and Polygons Page 21 of 21 The interior angle sum of a polygon. This general fact applies to all polygons and not just regular ones. Any polygon can similarly be split up onto triangles as shown below: There is an obvious pattern here any polygon with n sides can be split into n-2 triangles. Since the interior angle sum of a triangle is 180, the sum of the interior angles of a polygon can be given by the formula Angle sum = 180(n -2) where n is the number of the sides in the polygon. Example (6): A regular polygon has an interior angle sum of How many sides does it have? We must solve the equation: 180(n - 2) = Dividing by 180 on both sides, we have n - 2 = 8 and hence n = 10. The polygon in question is therefore 10-sided, i.e. a regular decagon.

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