Two straight roads leave a junction at an angle of 120° to each other. One car drives 30 km along one road while a second car drives 50 km along the other. The distance between the two cars is then:

By the cosine rule $d^2=30^2+50^2-2(30)(50)$·cos120°. Since cos120° = −1/2 the last term is +1500, so $d^2=3400+1500=4900$ and $d=70$ km. Taking cos120° as +1/2 gives √1900 ≈ 43.6 km, while 80 km and 20 km simply add or subtract the two distances as if the roads were straight in line.