Cosine rule
Applications of trigonometry · Mathematics
Study notes
Q: Two sides 5, 7 with included angle 60°. Find the third side. Then: sides 3,4,5 - is it right-angled? c² = 25 + 49 - 2(5)(7)cos60° = 74 - 35 = 39. c = √39. Check 3,4,5: cosC = (9+16-25)/24 = 0, so C = 90°. Yes, right-angled.