Mathematics for IGCSE & O level - Pythagoras' Theorem And Trigonometry (Section 1 - No. 23)
A boat sails 10 km due north and then 8 km in a south-easterly direction. To determine the final position of the boat you will need to consider:
Pythagoras' theorem
The sine rule
Trigonometry and vector addition
Only the sine rule
Explication
You'll need trigonometry (to deal with the angles) and vector addition (to combine the directions and distances).