Topics · Edexcel GCSE Maths (1MA1)
The cosine rule is Pythagoras with a correction for angles that are not 90°. Try the question, then see the correction change.
In triangle PQR, PQ = 8 cm, PR = 11 cm and angle QPR = 40°.
Work out the length of QR.
Give your answer correct to 3 significant figures.
See why
When angle P is bigger than 90°, is QR longer or shorter than √185?
Predict first:
Angle P: 40°QR: 7.08 cm√185: 13.60 cm (Pythagoras, only at 90°)
cos 40° is positive, so QR² = 185 − 134.8.
Now a fresh one
A triangle has sides of length 5 cm, 7 cm and 9 cm.
Work out the size of the largest angle in the triangle.
Give your answer correct to 1 decimal place.
Learn it properly
Where it comes up in the exam
- Specification G22 (Higher content): know and apply the sine rule and cosine rule to find unknown lengths and angles.
- Higher tier only. The formulae sheet gives the cosine rule, so the marks are for choosing it and using it correctly.
- Pearson's examiners report a trigonometry rule applied to the wrong triangle.