Using the cosine ratio
I can use the cosine ratio to find the missing side or angle in a right-angled triangle.
Using the cosine ratio
I can use the cosine ratio to find the missing side or angle in a right-angled triangle.
These resources will be removed by end of Summer Term 2025.
Lesson details
Key learning points
- The cosine ratio involves the hypotenuse, adjacent and the angle.
- If you know the length of the hypotenuse and the size of the angle, you can use the cosine ratio.
- If you know the length of the adjacent and the size of the angle, you can use the cosine ratio.
- If you know the length of the hypotenuse and adjacent, you can use the cosine ratio.
Keywords
Hypotenuse - The hypotenuse is the side of a right-angled triangle which is opposite the right angle.
Adjacent - The adjacent side of a right-angled triangle is the side which is next to both the right angle and the marked angle.
Trigonometric functions - Trigonometric functions are commonly defined as ratios of two sides of a right-angled triangle containing the angle.
Cosine function - The cosine of an angle (cos(θ°)) is the x-coordinate of point P on the triangle formed inside the unit circle.
Common misconception
The cosine formula is only used to find the length of a side adjacent to an angle.
The cosine formula can be used to find the length of a side adjacent to an angle. A rearrangement of the formula also allows us to find the length of the hypotenuse given the adjacent side. The arccosine function allows us to find the angle, itself.
To help you plan your year 9 maths lesson on: Using the cosine ratio, download all teaching resources for free and adapt to suit your pupils' needs...
To help you plan your year 9 maths lesson on: Using the cosine ratio, download all teaching resources for free and adapt to suit your pupils' needs.
The starter quiz will activate and check your pupils' prior knowledge, with versions available both with and without answers in PDF format.
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Explore more key stage 3 maths lessons from the Trigonometry unit, dive into the full secondary maths curriculum, or learn more about lesson planning.
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Starter quiz
6 Questions
Exit quiz
6 Questions
