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1-Sep-14Created by Mr. Lafferty Maths Dept. Trigonometry www.mathsrevision.com Cosine Rule Finding a Length Sine Rule Finding a length Mixed Problems S4.

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Presentation on theme: "1-Sep-14Created by Mr. Lafferty Maths Dept. Trigonometry www.mathsrevision.com Cosine Rule Finding a Length Sine Rule Finding a length Mixed Problems S4."— Presentation transcript:

1 1-Sep-14Created by Mr. Lafferty Maths Dept. Trigonometry Cosine Rule Finding a Length Sine Rule Finding a length Mixed Problems S4 Credit Sine Rule Finding an Angle Cosine Rule Finding an Angle Area of ANY Triangle

2 1-Sep-14Created by Mr. Lafferty Maths Dept. Starter Questions Starter Questions S4 Credit

3 1-Sep-14Created by Mr. Lafferty Maths Dept. Learning Intention Success Criteria 1.Know how to use the sine rule to solve REAL LIFE problems involving lengths. 1. To show how to use the sine rule to solve REAL LIFE problems involving finding the length of a side of a triangle. Sine Rule S4 Credit

4 C B A 1-Sep-14Created by Mr Lafferty Maths Dept Sine Rule S4 Credit a b c The Sine Rule can be used with ANY triangle as long as we have been given enough information. Works for any Triangle

5 Deriving the rule B C A b c a Consider a general triangle ABC. The Sine Rule Draw CP perpendicular to BA P This can be extended to or equivalently

6 Calculating Sides Using The Sine Rule 10m 34 o 41 o a Match up corresponding sides and angles: Rearrange and solve for a. Example 1 : Find the length of a in this triangle. S4 Credit A B C

7 Calculating Sides Using The Sine Rule S4 Credit 10m 133 o 37 o d = 12.14m Match up corresponding sides and angles: Rearrange and solve for d. Example 2 : Find the length of d in this triangle. C D E

8 What goes in the Box ? Find the unknown side in each of the triangles below: (1) 12cm 72 o 32 o a (2) 93 o b 47 o 16mm A = 6.7cm B = 21.8mm S4 Credit 1-Sep-14Created by Mr Lafferty Maths Dept

9 1-Sep-14Created by Mr. Lafferty Maths Dept. Now try MIA Ex 2.1 Ch12 (page 247) S4 Credit Sine Rule

10 1-Sep-14Created by Mr. Lafferty Maths Dept. Starter Questions Starter Questions S4 Credit

11 1-Sep-14Created by Mr. Lafferty Maths Dept. Learning Intention Success Criteria 1.Know how to use the sine rule to solve problems involving angles. 1. To show how to use the sine rule to solve problems involving finding an angle of a triangle. Sine Rule S4 Credit

12 Calculating Angles Using The Sine Rule Example 1 : Find the angle A o A 45m 23 o 38m Match up corresponding sides and angles: Rearrange and solve for sin A o = Use sin to find A o S4 Credit B C

13 Calculating Angles Using The Sine Rule 143 o 75m 38m X = Example 2 : Find the angle X o Match up corresponding sides and angles: Rearrange and solve for sin X o Use sin to find X o S4 Credit Y Z

14 What Goes In The Box ? Calculate the unknown angle in the following: (1) 14.5m 8.9m AoAo 100 o (2) 14.7cm BoBo 14 o 12.9cm A o = 37.2 o B o = 16 o S4 Credit

15 1-Sep-14Created by Mr. Lafferty Maths Dept. Now try MIA Ex3.1 Ch12 (page 249) S4 Credit Sine Rule

16 1-Sep-14Created by Mr. Lafferty Maths Dept. Starter Questions Starter Questions S4 Credit

17 1-Sep-14Created by Mr. Lafferty Maths Dept. Learning Intention Success Criteria 1.Know when to use the cosine rule to solve problems. 1. To show when to use the cosine rule to solve problems involving finding the length of a side of a triangle. Cosine Rule S4 Credit 2. Solve problems that involve finding the length of a side.

18 C B A 1-Sep-14Created by Mr Lafferty Maths Dept Cosine Rule S4 Credit a b c The Cosine Rule can be used with ANY triangle as long as we have been given enough information. Works for any Triangle

19 Deriving the rule A B C a b c Consider a general triangle ABC. We require a in terms of b, c and A. Draw BP perpendicular to AC b P x b - x BP 2 = a 2 – (b – x) 2 Also: BP 2 = c 2 – x 2  a 2 – (b – x) 2 = c 2 – x 2  a 2 – (b 2 – 2bx + x 2 ) = c 2 – x 2  a 2 – b 2 + 2bx – x 2 = c 2 – x 2  a 2 = b 2 + c 2 – 2bx*  a 2 = b 2 + c 2 – 2bcCosA *Since Cos A = x/c  x = cCosA When A = 90 o, CosA = 0 and reduces to a 2 = b 2 + c 2 1 When A > 90 o, CosA is negative,  a 2 > b 2 + c 2 2 When A b 2 + c 2 3 The Cosine Rule The Cosine Rule generalises Pythagoras’ Theorem and takes care of the 3 possible cases for Angle A. a 2 > b 2 + c 2 a 2 < b 2 + c 2 a 2 = b 2 + c 2 A A A Pythagoras + a bit Pythagoras - a bit Pythagoras

20 a 2 = b 2 + c 2 – 2bcCosA Applying the same method as earlier to the other sides produce similar formulae for b and c. namely: b 2 = a 2 + c 2 – 2acCosB c 2 = a 2 + b 2 – 2abCosC A B C a b c The Cosine Rule The Cosine rule can be used to find: 1. An unknown side when two sides of the triangle and the included angle are given (SAS). 2. An unknown angle when 3 sides are given (SSS). Finding an unknown side.

21 1-Sep-14Created by Mr Lafferty Maths Dept Cosine Rule S4 Credit How to determine when to use the Cosine Rule. Works for any Triangle 1. Do you know ALL the lengths. 2. Do you know 2 sides and the angle in between. SAS OR If YES to any of the questions then Cosine Rule Otherwise use the Sine Rule Two questions

22 Using The Cosine Rule Example 1 : Find the unknown side in the triangle below: L 5m 12m 43 o Identify sides a,b,c and angle A o a =Lb =5c =12A o =43 o Write down the Cosine Rule. a 2 =b2b2 +c2c2 -2bccosA o Substitute values to find a 2. a 2 = x 5 x 12 cos 43 o a 2 = (120 x0.731 ) a 2 =81.28 Square root to find “a”. a = L = 9.02m S4 Credit Works for any Triangle

23 Example 2 : Find the length of side M. 137 o 17.5 m 12.2 m M Identify the sides and angle. a = Mb = 12.2C = 17.5A o = 137 o Write down Cosine Rule a 2 =b2b2 +c2c2 -2bccosA o a 2 = – ( 2 x 12.2 x 17.5 x cos 137 o ) a 2 = – ( 427 x – ) Notice the two negative signs. a 2 = a 2 = a = M = 27.7m Using The Cosine Rule S4 Credit Works for any Triangle

24 What Goes In The Box ? Find the length of the unknown side in the triangles: (1) 78 o 43cm 31cm L (2) 8m 5.2m 38 o M L = 47.5cm M =5.05m S4 Credit

25 1-Sep-14Created by Mr. Lafferty Maths Dept. Now try MIA Ex4.1 Ch12 (page 254) S4 Credit Cosine Rule

26 1-Sep-14Created by Mr. Lafferty Maths Dept. Starter Questions Starter Questions S4 Credit 54 o

27 1-Sep-14Created by Mr. Lafferty Maths Dept. Learning Intention Success Criteria 1.Know when to use the cosine rule to solve problems. 1.Know when to use the cosine rule to solve REAL LIFE problems. 1. To show when to use the cosine rule to solve REAL LIFE problems involving finding an angle of a triangle. Cosine Rule S4 Credit 2. Solve problems that involve finding an angle of a triangle. 2. Solve REAL LIFE problems that involve finding an angle of a triangle.

28 C B A 1-Sep-14Created by Mr Lafferty Maths Dept Cosine Rule S4 Credit a b c The Cosine Rule can be used with ANY triangle as long as we have been given enough information. Works for any Triangle

29 Finding Angles Using The Cosine Rule Consider the Cosine Rule again: a 2 =b2b2 +c2c2 -2bccosA o We are going to change the subject of the formula to cos A o Turn the formula around: b 2 + c 2 – 2bc cos A o = a 2 Take b 2 and c 2 across. -2bc cos A o = a 2 – b 2 – c 2 Divide by – 2 bc. Divide top and bottom by -1 You now have a formula for finding an angle if you know all three sides of the triangle. S4 Credit Works for any Triangle

30 AoAo 16cm 9cm11cm Write down the formula for cos A o Label and identify A o and a, b and c. A o = ? a = 11b = 9c = 16 Substitute values into the formula. Calculate cos A o. Cos A o =0.75 Use cos to find A o A o = 41.4 o Example 1 : Calculate the unknown angle A o. Finding Angles Using The Cosine Rule S4 Credit Works for any Triangle

31 Example 2: Find the unknown Angle y o in the triangle: 26cm 15cm 13cm yoyo Write down the formula. Identify the sides and angle. A o = y o a = 26b = 15c = 13 Find the value of cosA o cosA o = The negative tells you the angle is obtuse. A o = y o =136.3 o S4 Credit Finding Angles Using The Cosine Rule Works for any Triangle

32 What Goes In The Box ? Calculate the unknown angles in the triangles below: (1) 10m 7m 5m AoAo BoBo (2) 12.7cm 7.9cm 8.3cm A o =111.8 o B o = 37.3 o S4 Credit

33 1-Sep-14Created by Mr. Lafferty Maths Dept. Now try MIA Ex 5.1 Ch12 (page 256) S4 Credit Cosine Rule

34 1-Sep-14Created by Mr. Lafferty Maths Dept. Starter Questions Starter Questions S4 Credit

35 1-Sep-14Created by Mr. Lafferty Maths Dept. Learning Intention Success Criteria 1.Know the formula for the area of any triangle. 1. To explain how to use the Area formula for ANY triangle. S4 Credit Area of ANY Triangle 2.Use formula to find area of any triangle given two length and angle in between.

36 1-Sep-14Created by Mr Lafferty Maths Dept Labelling Triangles S4 Credit A B C A a B b C c Small letters a, b, c refer to distances Capital letters A, B, C refer to angles In Mathematics we have a convention for labelling triangles.

37 F E D F E D 1-Sep-14Created by Mr Lafferty Maths Dept Labelling Triangles S4 Credit d e f Have a go at labelling the following triangle.

38 General Formula for Area of ANY Triangle Consider the triangle below: AoAo BoBo CoCo a b c h Area = ½ x base x height What does the sine of A o equal Change the subject to h. h = b sinA o Substitute into the area formula S4 Credit

39 1-Sep-14Created by Mr Lafferty Maths Dept Area of ANY Triangle S4 Credit A B C A a B b C c The area of ANY triangle can be found by the following formula. Another version Another version Key feature To find the area you need to knowing 2 sides and the angle in between (SAS)

40 1-Sep-14Created by Mr Lafferty Maths Dept Area of ANY Triangle S4 Credit A B C A 20cm B 25cm C c Example : Find the area of the triangle. The version we use is 30 o

41 1-Sep-14Created by Mr Lafferty Maths Dept Area of ANY Triangle S4 Credit D E F 10cm 8cm Example : Find the area of the triangle. The version we use is 60 o

42 What Goes In The Box ? Calculate the areas of the triangles below: (1) 23 o 15cm 12.6cm (2) 71 o 5.7m 6.2m A = 36.9cm 2 A = 16.7m 2 S4 Credit Key feature Remember (SAS)

43 1-Sep-14Created by Mr. Lafferty Maths Dept. Now try MIA Ex6.1 Ch12 (page 258) S4 Credit Area of ANY Triangle

44 1-Sep-14Created by Mr. Lafferty Maths Dept. Starter Questions Starter Questions S4 Credit 61 o

45 1-Sep-14Created by Mr. Lafferty Maths Dept. Learning Intention Success Criteria 1.Be able to recognise the correct trigonometric formula to use to solve a problem involving triangles. 1. To use our knowledge gained so far to solve various trigonometry problems. Mixed problems S4 Credit

46 SOH CAH TOASOH CAH TOA 25 o 15 m A D The angle of elevation of the top of a building measured from point A is 25 o. At point D which is 15m closer to the building, the angle of elevation is 35 o Calculate the height of the building. T B Angle TDA = 145 o Angle DTA = 10 o 35 o – 35 = 145 o 180 – 170 = 10 o Exam Type Questions

47 A fishing boat leaves a harbour (H) and travels due East for 40 miles to a marker buoy (B). At B the boat turns left and sails for 24 miles to a lighthouse (L). It then returns to harbour, a distance of 57 miles. (a)Make a sketch of the journey. (b)Find the bearing of the lighthouse from the harbour. (nearest degree) H 40 miles 24 miles B L 57 miles A Exam Type Questions

48 A The angle of elevation of the top of a column measured from point A, is 20 o. The angle of elevation of the top of the statue is 25 o. Find the height of the statue when the measurements are taken 50 m from its base 50 m Angle BCA = 70 o Angle ACT = Angle ATC = 110 o 65 o m B T C 180 – 110 = 70 o 180 – 70 = 110 o 180 – 115 = 65 o 20 o 25 o 5o5o SOH CAH TOASOH CAH TOA Exam Type Questions

49 An AWACS aircraft takes off from RAF Waddington (W) on a navigation exercise. It flies 530 miles North to a point (P) as shown, It then turns left and flies to a point (Q), 670 miles away. Finally it flies back to base, a distance of 520 miles. Find the bearing of Q from point P. P 670 miles W 530 miles Not to Scale Q 520 miles Exam Type Questions

50 1-Sep-14Created by Mr. Lafferty Maths Dept. Now try MIA Ex 7.1 & 7.2 Ch12 (page 262) S4 Credit Mixed Problems


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