The Law of Cosines
This is The Law of Cosines (also called the Cosine Rule):
It works for any triangle:
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a, b and c are sides.
C is the angle opposite side c |
Let's see how to use it in an example:
Example: How long is side "c" ... ?
We know angle C = 37º, a = 8 and b = 11
| The Law of Cosines says: |
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c2 = a2 + b2 - 2ab cos(C) |
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| Put in the values we know: |
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c2 = 82 + 112 - 2 × 8 × 11 × cos(37º) |
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| Do some calculations: |
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c2 = 64 + 121 - 176 × 0.798… |
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| Which gives us: |
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c2 = 44.44... |
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| Take the square root: |
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c = √44.44 = 6.67 (to 2 decimal places) |
Answer: c = 6.67
How to Remember
How can you remember the formula?
Well, it helps to know that it is the Pythagoras Theorem with something extra so it works for all triangles:
| Pythagoras Theorem: |
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a2 + b2 = c2 |
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(only for Right-Angled Triangles) |
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| Law of Cosines: |
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a2 + b2 - 2ab cos(C) = c2 |
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(for all triangles) |
So, to remember it:
- think "abc": a2 + b2 = c2,
- then another "abc": 2ab cos(C),
- and put them together: a2 + b2 - 2ab cos(C) = c2
When to Use
The law of cosines is useful for finding:
- the third side of a triangle when you know two sides and the angle between them (like the example above)
- the angles of a triangle when you know all three sides (as in the following example)
Example: What is Angle "C" ...?
The side of length "8" is opposite angle C, so it is side c. The other two sides are a and b.
Now let us put what we know into The Law of Cosines:
| Start with: |
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c2 = a2 + b2 - 2ab cos(C) |
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| Put in a, b and c |
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82 = 92 + 52 - 2 × 9 × 5 × cos(C) |
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| Calculate: |
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64 = 81 + 25 - 90 × cos(C) |
| Calculate some more: |
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64 = 106 - 90 × cos(C) |
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| Now we use our algebra skills to rearrange and solve: |
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| Subtract 64 from both sides: |
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0 = 42 - 90 × cos(C) |
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| Add "90 × cos(C)" to both sides: |
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90 × cos(C) = 42 |
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| Divide both sides by 90: |
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cos(C) = 42/90 |
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| Inverse cosine: |
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C = cos-1(42/90) |
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| Calculator: |
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C = 62.2° (to 1 decimal place) |
In Other Forms
Easier Version For Angles
There is a version that is easier to use when finding angles. It is simply a rearrangement of the c2 = a2 + b2 - 2ab cos(C) formula like this:
Example: Find Angle "C"
In this triangle we know the three sides:
Use The Law of Cosines (angle version) to find angle C :
cos C = (a² + b² - c²)/2ab
= (8² + 6² - 7²)/2×8×6
= (64 + 36 - 49)/96
= 51/96
= 0.53125
C = cos-1(0.53125)
= 57.9° correct to one decimal place.
Versions for a, b and c
Also, you can rewrite the c2 = a2 + b2 - 2ab cos(C) formula into "a2=" and "b2=" form.
Here are all three:
But it is easier to remember the "c2=" form and change the letters as needed !
As in this example:
Example: Find the distance "z"
The letters are different! But that doesn't matter. We can easily substitute x for a, y for b and z for c
| Start with: |
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c2 = a2 + b2 - 2ab cos(C) |
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| x for a, y for b and z for c |
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z2 = x2 + y2 - 2xy cos(Z) |
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| Put in the values we know: |
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z2 = 9.42 + 6.52 - 2×9.4×6.5×cos(131º) |
| Calculate: |
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z2 = 88.36 + 42.25 - 122.2×(-0.656...) |
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z2 = 130.61 + 80.17... |
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z2 = 210.78... |
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z = √210.78... = 14.5 to 1 decimal place. |
Answer: z = 14.5
Did you notice that cos(131º) is negative and this changes the last sign in the calculation to + (plus)? The cosine of an obtuse angle is always negative (see Unit Circle).
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