1. GCSE Trigonometry Part 1 – Right-Angled Triangles
Dr J Frost
Objectives:
Recap of finding missing sides and angles in right-angled triangles.
Deal with more complicated sides, e.g. surds.
3D Trigonometry and Pythagoras.
Last modified: 23rd November 2016

2. 𝑓 𝑥 =2𝑥+1 5 11 RECAP: What is sin/cos/tan? ×2+1 input output
You may be familiar with the idea of a ‘number machine’, which takes an input, does something with it, and gives you an output.
The proper name for this is a ‘function’.
name of function
input
output
𝑓 𝑥 =2𝑥+1
We’ll be exploring functions in detail later in the GCSE syllabus.

3. 𝜃 RECAP: What is sin/cos/tan? 𝑠𝑖𝑛 tan 45 =1 ? = 𝑜 ℎ ℎ 𝑜 𝜃 𝑎 45° input
“opposite” to the angle
input
output
hypotenuse ℎ 𝜃
𝑠𝑖𝑛
Ratio between opposite and hypotenuse
= 𝑜 ℎ 𝑜 𝜃 𝑎
“adjacent” to the angle
Sin, cos and tan are functions which take an angle as an input, and gives the ratio between the lengths of different pairs of sides in a right-angled triangle.
For example, sin will take an angle, and (as the output) tell you how many times bigger the opposite is than the hypotenuse, i.e. 𝑜 ℎ
When the input of tan is 45, the output will be “how many times bigger is the opposite than the adjacent”? !
sin 𝜃 = 𝑜 ℎ cos 𝜃 = 𝑎 ℎ tan 𝜃 = 𝑜 𝑎
Remember as “soh cah toa”
tan 45 =1 ?
45°

4. RECAP: Finding missing sides angles
Bro Tip: Label any used sides first (I circle to avoid confusion with side lengths) 𝑦 5 4 h o
31°
25° 𝑥 a ?
“swapsie trick”!
i.e. we can swap the thing we’re dividing by and the result
tan 31° = 4 𝑦 𝑦= 4 tan 31 =6.66
cos 25° = 𝑥 5 𝑥=5 cos 25 =4.53 ? ?
Start of the same way as before, i.e. “ sin 𝑎𝑛𝑔𝑙𝑒 = 𝑠𝑖𝑑𝑒 𝑠𝑖𝑑𝑒 ” 5
sin 𝜃 = 3 5 𝜃= sin − =36.87° 3 𝜃
We want to make 𝜃 the subject, but there’s a sin on front to get rid of.
𝑠𝑖𝑛 −1 is the opposite of sin. Therefore we “sin-1” each side which undoes the sin.

5. Test Your Understanding𝑥 1 2 3
50°
26° 4 3 6 𝑦 𝜃 5
sin 26 = 3 𝑦 𝑦= 3 sin 26 =6.84 ? ?
tan 𝜃 = 4 5 𝜃= tan − =38.7° ?
cos 50 = 𝑥 6 𝑥=6 cos 50 =3.86 4 13 17 ?
𝑥= − =5 𝜃= sin − =17.1 𝜃 12

6. sin/cos/tan of 30°, 45°, 60°, 90°
You will frequently encounter angles of 30°, 60°, 45° in geometric problems. Why?
We see these angles in equilateral triangles and right-angled isosceles triangles. ?
You need to be able to calculate these in non-calculator exams.
All you need to remember:
! Draw half a unit square and half an equilateral triangle of side 2.
sin 30° = cos 30° = tan 30° = sin 60° = cos 60° = tan 60° = 3 ?
sin 45° = cos 45° = tan 45° =1 ? 2 ? ? ? 1 ? ?
30° ? 2 ? ?
45° 3 ? ? ? ? 1
60° ? 1 ? ? ?

7. Example Exam Questions
? Mark Scheme

8. Harder Further Maths Examples (including surds)1 2 𝐴 1 𝐵 𝜃 3
30° 𝑥 𝐶 𝐷
Find 𝑥.
𝐜𝐨𝐬 𝟑𝟎° = 𝒙 𝟒 𝟐 +𝟏𝟎 𝟑 𝟐 = 𝒙 𝟒 𝟐 +𝟏𝟎 𝟑 𝟒 𝟐 +𝟏𝟎 =𝟐𝒙 𝒙= 𝟑 𝟒 𝟐 +𝟏𝟎 𝟐 𝒙= 𝟒 𝟔 +𝟏𝟎 𝟑 𝟐 𝒙=𝟐 𝟔 +𝟓 𝟑
Determine cos 𝜃 𝐜𝐨𝐬 𝜽 = 𝟏 𝟑
Hence determine the length 𝐵𝐶. Using triangle 𝐴𝐶𝐷: 𝐜𝐨𝐬 𝜽 = 𝟑 𝑨𝑪 𝟏 𝟑 = 𝟑 𝑨𝑪 𝑨𝑪=𝟗 𝑩𝑪=𝟗−𝟏=𝟖 ? ?
If a fraction on each side (and nothing else), cross multiply. ?

9. Test Your Understanding
June 2013 Paper 1 Q3 2
𝐴𝐵𝐶 is a right-angled triangle. 𝑃 is a point on 𝐴𝐵.
tan 𝑥 = 2 3
2 2 −1
45° 𝑥
Find 𝑥.
𝐬𝐢𝐧 𝟒𝟓° = 𝒙 𝟐 𝟐 −𝟏 𝟏 𝟐 = 𝒙 𝟐 𝟐 −𝟏 𝟐 𝟐 −𝟏=𝒙 𝟐 𝒙= 𝟐 𝟐 −𝟏 𝟐 𝒙= 𝟒− 𝟐 𝟐 ?
a) Work out the length of 𝐵𝐶.
𝐭𝐚𝐧 𝒙 = 𝟒 𝑩𝑪 𝟐 𝟑 = 𝟒 𝑩𝑪 → 𝟐𝑩𝑪=𝟏𝟐, 𝑩𝑪=𝟔
b) Work out the length of 𝐴𝑃.
Using triangle 𝑨𝑩𝑪: 𝐭𝐚𝐧 𝒙 = 𝑩𝑪 𝑨𝑩 𝟐 𝟑 = 𝟔 𝑨𝑩 → 𝟐𝑨𝑩=𝟏𝟖 → 𝑨𝑩=𝟗 𝑨𝑷=𝟗−𝟒=𝟓𝒄𝒎 ?
Rationalise denominator by multiplying top and bottom by 2 . ?

10. Exercise 1 ? ? ? ? ? ? ? NO CALCULATORS 3 Show that 𝑝 is an integer. 4
Find the exact value of 𝑥. 2 1
𝒑=𝟔 ? 𝑥 𝑝 𝑥
5 2
45° 𝑥
30°
45°
60°
𝒙=𝟓 ?
8 + 2
𝒙=𝟓+ 𝟑 ?
𝒙=𝟑+𝟐 𝟑 ? 𝐶 4 𝐵 7
Determine the area of triangle 𝐴𝐵𝐶, giving your answer in the form 𝑝+𝑞 3 .
Work out the area of trapezium 𝑃𝑄𝑅𝑆 5 6 𝑃 12 𝑄
3+ 3 7 𝑥
30°
[AQA Mock Papers]
a) Using Δ𝐴𝐵𝐶, find tan 𝑥 . = 𝟏 𝟑
b) Work out the length of 𝑃𝑄 = 𝟕 𝟑 𝐴 ?
𝒑=𝟗+𝟔 𝟑 ? ?

11. 3D Pythagoras
The key here is identify some 2D triangle ‘floating’ in 3D space.
You will usually need to use Pythagoras twice.
Find the length of diagonal connecting two opposite corners of a unit cube, and the angle this makes with the horizontal plane. 2 3 ?
Using the base:
= 2
= 3
𝜃= tan − =35.3 1 1 1
Your Go…
Find the length of diagonal connecting two opposite corners of a unit cube, and the angle this makes with the horizontal plane. 12 ° ? 4 3

12. Further Example
Hint: You may want to use Pythagoras on the square base (drawing the diagonal in)
Determine the height of this pyramid. 2 2 ? 2 2 2
Your Go… 5
119 12 ?
119 8 6

13. Test Your Understanding
AQA Mock Set 4 Paper 2
AQA Mock Set 1 Paper 2
A pyramid has a square base 𝐴𝐵𝐶𝐷 of sides 6cm. Vertex 𝑉, is directly above the centre of the base, 𝑋.
Work out the height 𝑉𝑋 of the pyramid.
𝑿𝑨= 𝟏 𝟐 𝟔 𝟐 + 𝟔 𝟐 =𝟑 𝟐 𝑽𝑿= 𝟏𝟎 𝟐 − 𝟑 𝟐 𝟐 = 𝟏𝟎𝟎−𝟏𝟖 = 𝟖𝟐 2
The diagram shows a vertical mast, 𝐴𝐵, 12 metres high. Points 𝐵, 𝐶, 𝐷 are on a horizontal plane.
Point 𝐶 is due West of 𝐵.
Calculate 𝐶𝐷
Calculate the bearing of 𝐷 from 𝐶 to the nearest degree 1 ?
a) 𝑩𝑪= 𝟏𝟐 𝐭𝐚𝐧 𝟑𝟓 =𝟏𝟕.𝟏𝟑𝟕𝟕𝟕𝟔 𝑩𝑫= 𝟏𝟐 𝐭𝐚𝐧 𝟐𝟑° =𝟐𝟖.𝟐𝟕𝟎𝟐𝟐𝟖 𝑪𝑫= 𝟏𝟕.𝟏… 𝟐 + 𝟐𝟖.𝟐… 𝟐 =𝟑𝟑.𝟎𝟔𝒎
b) 𝟗𝟎°+ 𝐭𝐚𝐧 −𝟏 𝟐𝟖.𝟐𝟕 𝟏𝟕.𝟏𝟒 =𝟏𝟒𝟗° ? ?

14. Angles between lines and planes
A plane is:
A flat 2D surface (not necessary horizontal). ?
When we want to find the angle between a line and a plane, use the “drop method” – imagine the line is a pen which you drop onto the plane. The angle you want is between the original and dropped lines. 𝜃
Click for
Bromanimation

15. Angles between two planes
To find angle between two planes:
Use ‘line of greatest slope’* on one plane, before again ‘dropping’ down.
* i.e. what direction would be ball roll down? 𝜃
line of greatest slope ? 𝐵
Example:
Find the angle between the plane 𝐴𝐵𝐶𝐷 and the horizontal plane.
𝜽= 𝐬𝐢𝐧 −𝟏 𝟑 𝟔 =𝟑𝟎°
6𝑐𝑚 𝐶 𝜽
3𝑐𝑚
line of greatest slope ? 𝐴 𝐷

16. Further Example 2 2 2 2 1 𝐸 𝐷 𝐶 𝑂 ? 𝐴 𝐵 𝐸 ? Suitable diagram
Find the angle between the planes 𝐵𝐶𝐸 and 𝐴𝐵𝐶𝐷. 2 2
We earlier used Pythagoras to establish the height of the pyramid. Why would it be wrong to use the angle ∠𝐸𝐶𝑂?
𝑬𝑪 is not the line of greatest slope. 𝐷 𝐶 𝑂 2 ? 𝐴 2 𝐵 𝐸
? Suitable diagram
? Solution 2 𝐷 𝐶
𝜃= tan − =54.7° 𝜃 1 𝑂
From 𝐸 a ball would roll down to the midpoint of 𝐵𝐶. 𝐴 𝐵

17. Test Your Understanding
Jan 2013 Paper 2 Q23
The diagram shows a cuboid 𝐴𝐵𝐶𝐷𝑃𝑄𝑅𝑆 and a pyramid 𝑃𝑄𝑅𝑆𝑉. 𝑉 is directly above the centre 𝑋 of 𝐴𝐵𝐶𝐷.
a) Work out the angle between the line 𝑉𝐴 and the plane 𝐴𝐵𝐶𝐷. (Reminder: ‘Drop’ 𝑉𝐴 onto the plane) 𝑨𝑿= 𝟏 𝟐 𝟔 𝟐 + 𝟏𝟎 𝟐 = 𝟑𝟒 ∠𝑽𝑨𝑿= 𝐭𝐚𝐧 −𝟏 𝟓 𝟑𝟒 =𝟒𝟎.𝟔°
b) Work out the angle between the planes 𝑉𝑄𝑅 and 𝑃𝑄𝑅𝑆.
𝐭𝐚𝐧 −𝟏 𝟐 𝟓 =𝟐𝟏.𝟖° ? ?

18. Exercise 2 ? ? ? ? 𝐵 8 𝐶 8 𝐴 A cube has sides 8cm. Find: 1 2
a) The length 𝐴𝐵. 𝟖 𝟑
b) The angle between 𝐴𝐵 and the horizontal plane. 𝟑𝟓.𝟑° 1 2
A radio tower 150m tall has two support cables running 300m due East and some distance due South, anchored at 𝐴 and 𝐵. The angle of inclination to the horizontal of the latter cable is 50° as indicated. ? ? 𝐵 8
150𝑚 𝐶 𝐴
300𝑚
50° 8 𝐴 𝐵
a) Find the angle between the cable attached to 𝐴 and the horizontal plane. 𝐭𝐚𝐧 −𝟏 𝟏𝟓𝟎 𝟑𝟎𝟎 =𝟐𝟔.𝟔°
b) Find the distance between 𝐴 and 𝐵 m ? ?

19. Exercise 2 𝐸 𝐷 𝐶 𝑀 𝑋 𝐴 𝐵 ? ? ? ? 𝐼 𝐽 20cm 𝐺 𝐻 12𝑚 𝐹 𝐸 8𝑚 𝐷 24cm 𝐶 7𝑚 𝐴
24𝑚 7𝑚 𝐸 3 4
20cm 𝐷 𝐶 𝑀 𝑋
24cm 𝐴
24cm 𝐵
A school buys a set of new ‘extra comfort’ chairs with its seats pyramid in shape. 𝑋 is at the centre of the base of the pyramid, and 𝑀 is the midpoint of 𝐵𝐶.
By considering the triangle 𝐸𝐵𝐶, find the length 𝐸𝑀. 16cm
Hence determine the angle between the triangle 𝐸𝐵𝐶 and the plane 𝐴𝐵𝐶𝐷. 𝐜𝐨𝐬 −𝟏 𝟏𝟐 𝟏𝟔 =𝟒𝟏.𝟒°
Frost Manor is as pictured, with 𝐸𝐹𝐺𝐻 horizontally level.
a) Find the angle between the line 𝐴𝐺 and the plane 𝐴𝐵𝐶𝐷. 𝐭𝐚𝐧 −𝟏 𝟖 𝟐𝟓 =𝟏𝟕.𝟕°
b) Find the angle between the planes 𝐹𝐺𝐼𝐽 and 𝐸𝐹𝐺𝐻. 𝐭𝐚𝐧 −𝟏 𝟒 𝟏𝟐 =𝟏𝟖.𝟒° ? ? ? ?

20. Exercise 2 5
[June 2013 Paper 2] 𝐴𝐵𝐶𝐷𝐸𝐹𝐺𝐻 is a cuboid. 𝑀 is the midpoint of 𝐻𝐺. 𝑁 is the midpoint of 𝐷𝐶.
Show that 𝐵𝑁= 7.5m
Work out the angle between the line 𝑀𝐵 and the plane 𝐴𝐵𝐶𝐷. 𝒕𝒂𝒏 −𝟏 𝟑 𝟕.𝟓 =𝟐𝟏.𝟖°
Work out the obtuse angle between the planes 𝑀𝑁𝐵 and 𝐶𝐷𝐻𝐺. 𝟏𝟖𝟎− 𝐭𝐚𝐧 −𝟏 𝟒.𝟓 𝟔 =𝟏𝟒𝟑.𝟏° 6
[Set 3 Paper 2] The diagram shows part of a skate ramp, modelled as a triangular prism.
𝐴𝐵𝐶𝐷 represents horizontal ground. The vertical rise of the ramp, 𝐶𝐹, is 7 feet. The distance 𝐵𝐶=24 feet. ? ?
You are given that 𝑔𝑟𝑎𝑑𝑖𝑒𝑛𝑡= 𝑣𝑒𝑟𝑡𝑖𝑐𝑎𝑙 𝑟𝑖𝑠𝑒 ℎ𝑜𝑟𝑖𝑧𝑜𝑛𝑡𝑎𝑙 𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒
a) The gradient of 𝐵𝐹 is twice the gradient of 𝐴𝐹. Write down the distance 𝐴𝐶
b) Greg skates down the ramp along 𝐹𝐵. How much further would he travel if he had skated along 𝐹𝐴. 𝑭𝑩=𝟐𝟓 𝑨𝑭=𝟒𝟖.𝟓𝟏 𝟒𝟖.𝟓𝟏−𝟐𝟓=𝟐𝟑.𝟓𝟏 ? ?

21. Exercise 2 ? ? ? 𝐼 2 2 𝐺 3 𝐻 5𝑚 𝐹 3 𝐸 2𝑚 𝐷 4 𝐶 4 4𝑚 𝐴 8𝑚 𝐵 7 8
A ‘truncated pyramid’ is formed by slicing off the top of a square-based pyramid, as shown. The top and bottom are two squares of sides 2 and 4 respectively and the slope height 3.
Find the angle between the sloped faces with the bottom face.
𝐜𝐨𝐬 −𝟏 𝟏 𝟖 =𝟔𝟗.𝟑°
a) Determine the angle between the line 𝐴𝐼 and the plane 𝐴𝐵𝐶𝐷.
𝐭𝐚𝐧 −𝟏 𝟓 𝟐 𝟓 =𝟒𝟖.𝟐°
b) Determine the angle between the planes 𝐹𝐻𝐼 and 𝐸𝐹𝐺𝐻.
𝐭𝐚𝐧 −𝟏 𝟑 𝟒 =𝟑𝟔.𝟗° ? ? ?

22. Exercise 2 𝐶 N 5 𝐷 4 𝐵 3 𝑋 𝐴
a) 𝑋 is a point on 𝐴𝐵 such that 𝑋𝐶 is the line of greatest slope on the triangle 𝐴𝐵𝐶. Determine the length of 𝐴𝑋.
By Pythagoras 𝑨𝑪= 𝟑𝟒 , 𝑩𝑪= 𝟒𝟏 and 𝑨𝑩=𝟓. If 𝑨𝑿=𝒙, then 𝑿𝑩=𝟓−𝒙. Then 𝑪𝑿 can be expressed in two different ways:
𝑪𝑿= 𝟑𝟒− 𝒙 𝟐 = 𝟒𝟏− 𝟓−𝒙 𝟐
Solving: 𝑪𝑿= 𝟒 𝟓
b) Hence determine 𝐷𝑋. 𝟑𝟒− 𝟎.𝟖 𝟐 = 𝟖𝟑𝟒 𝟓 =𝟓.𝟕𝟕𝟔
c) Hence find the angle between the planes 𝐴𝐵𝐶 and 𝐴𝐵𝐷. 𝐭𝐚𝐧 −𝟏 𝟓 𝟓.𝟕𝟕𝟔 =𝟒𝟎.𝟖𝟖 ? ? ?