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If the question concerns lengths or angles in a triangle, you may need the sine rule or the cosine rule.
First, decide if the triangle is right-angled.
Then, decide whether an angle is involved at all.
If it is a right-angled triangle, and there are angles involved, you will need straightforward Trigonometry, using Sin, Cos and Tan.
If the triangle is not right-angled, you may need the Sine Rule or the Cosine Rule
If it is a right-angled triangle, and there are no angles involved, you will need Pythagoras’ Theorem
In any triangle ABC
The Sine Rule:
A B
C
ab
c
C
c
B
b
A
a
sinsinsin
or
c
C
b
B
a
A sinsinsin
Not right-angled!
You do not have to learn the Sine Rule or the Cosine Rule!
They are always given to you at the front of the Exam Paper.
You just have to know when and how to use them!
The Sine Rule:
A B
C
ab
c
You can only use the Sine Rule if you have a “matching pair”.
You have to know one angle, and the side opposite it.
The Sine Rule:
A B
C
ab
c
You can only use the Sine Rule if you have a “matching pair”.
You have to know one angle, and the side opposite it.
Then if you have just one other side or angle, you can use the Sine Rule to find any of the other angles or sides.
10cm
x
65°
Finding the missing side:
Is it a right-angled triangle?
Is there a matching pair?
No
Yes
40°
Not to scale
10cm
65°
Finding the missing side:
Is it a right-angled triangle?
Is there a matching pair?
No
Yes
Label the sides and angles.
A
B
C
ab
c
40°x
Use the Sine Rule
Not to scale
10cm
65°
Finding the missing side:
A
B
C
ab
c
40°x
C
c
B
b
A
a
sinsinsin
We don’t need the “C” bit of the formula.
Because we are trying to find a missing length of a side, the little letters are on top
Not to scale
10cm
65°
Finding the missing side:
A
B
C
ab
c
40°x
B
b
A
a
sinsin
Fill in the bits you know.
Because we are trying to find a missing length of a side, the little letters are on top
Not to scale
10cm
65°
Finding the missing side:
A
B
C
ab
c
40°x
B
b
A
a
sinsin
Fill in the bits you know.
65sin
10
40sin
x
Not to scale
10cm
65°
Finding the missing side:
A
B
C
ab
c
40°x
B
b
A
a
sinsin
65sin
10
40sin
x
40sin65sin
10x
09.7x cm
Not to scale
10cm
7.1cm
65°
Finding the missing angle:
Is it a right-angled triangle?
Is there a matching pair?
No
Yes
θ°
Not to scale
Finding the missing angle:
Is it a right-angled triangle?
Is there a matching pair?
No
Yes
Label the sides and angles.
A
B
C
ab
c
Use the Sine Rule
10cm
7.1cm
65°
θ°
Not to scale
Finding the missing angle:
We don’t need the “C” bit of the formula.
A
B
C
ab
c
10cm
7.1cm
65°
θ°
Because we are trying to find a missing angle, the formula is the other way up. c
C
b
B
a
A sinsinsin
Not to scale
Finding the missing angle:
Fill in the bits you know.
Because we are trying to find a missing angle, the formula is the other way up.
A
B
C
ab
c
10cm
7.1cm
65°
θ°
b
B
a
A sinsin
Not to scale
Finding the missing angle:
Fill in the bits you know.
10
65sin
1.7
sin
A
B
C
ab
c
10cm
7.1cm
65°
θ°
b
B
a
A sinsin
Not to scale
Finding the missing angle:
1.710
65sinsin
.....6434785.0sin
A
B
C
ab
c
10cm
7.1cm
65°
θ°
10
65sin
1.7
sin
05.40Shift Sin =
b
B
a
A sinsin
Not to scale
If the triangle is not right-angled, and there is not a matching pair, you will need the Cosine Rule.
The Cosine Rule:
A B
C
ab
c
In any triangle ABC Abccba cos2222
Finding the missing side:
Is it a right-angled triangle?
Is there a matching pair?
No
No
9cm
12cm
20°A
C
B
x
Use the Cosine Rule
Label the sides and angles, calling the given angle “A” and the missing side “a”.
ab
c Not to scale
Finding the missing side:
9cm
12cm
20°A
C
B
x ab
c
Fill in the bits you know.
Abccba cos2222
x = 4.69cm
20cos9122912 222a
)20cos9122(912 222 a
........026.22a69.4a
Not to scale
.......026.222 a
Finding the missing side:
Is it a right-angled triangle?
Is there a matching pair?
No
No
8km
5km
130°
A man starts at the village of Chartham and walks 5 km due South to Aylesham. Then he walks another 8 km on a bearing of 130° to Barham.
What is the direct distance between Chartham and Barham, in a straight line?
A
C
B
First, draw a sketch.
Use the Cosine Rule
Not to scale
Finding the missing side:
a
8km
5km
130°
A man starts at the village of Chartham and walks 5 km due South to Aylesham. Then he walks another 8 km to on a bearing of 130° to Barham.
What is the direct distance between Chartham and Barham, in a straight line?
A
C
B
Abccba cos2222 Call the missing length you want to find “a”Label the other sides
b
ca² = 5² + 8² - 2 x 5 x 8 x cos130°
a² = 25 + 64 - 80cos130°
a² = 140.42
a = 11.85 11.85km
Not to scale
Is it a right-angled triangle?
Is there a matching pair?
No
No
Use the Cosine Rule
a9cm6cm
A
C
B
b
c10cmθ°
Label the sides and angles, calling the missing angle “A”
Finding the missing angle θ:
Not to scale
Finding the missing angle θ:
a9cm6cm
A
C
B
b
c10cmθ°
Abccba cos2222
Not to scale
This can be rearranged to:
bc
acbA
2cos
222
1062
9106cos
222
A
458333333.0cos A
Shift Cos =
7.62)458333333.0(cos 1
72.62
The diagram shows the route taken of an orienteering competition.The shape of the course is a quadrilateral ABCD.AB = 4 km, BC = 4.5 km and CD = 5 km.The angle at B is 70° and the angle at D is 52°
Calculate the angle CAD.
4km 4.5km
A
5kmNot to scale
B
C
D
70°
52°
θ°x
We haven’t got enough information
about the triangle ACD to find
First find x, by looking at just the upper triangle, because this will give us enough information in the lower triangle to find .
4km 4.5km
A
B
C
70°
x
Is it a right-angled triangle?
Is there a matching pair?
No
No
Use the Cosine Rule
Label the sides and angles, calling the given angle “A” and the missing side “a”.
A
a
B
b
C
c
4km 4.5km
A
B
C
70°
x
A
a
B
b
C
c
Abccba cos2222 70cos45.4245.4 222 a
93727484.232 a
93727484.23a
892573437.4ax = 4.893km (to 3 d.p.)
A
5km
C
D
52°
θ°4.893km
Now look at the lower triangle
Is it a right-angled triangle?
Is there a matching pair?
No
Yes
Label the sides and angles.
Use the Sine Rule
B
b
a
A
A
5km
C
D
52°
θ°4.893km
B
b
b
B
a
A sinsin
893.4
52sin
5
sin
5893.4
52sinsin
805242952.0sin
6.53Shift Sin =
A
a