AD=2/3AB=2/3(a-b)

CD = AD-AC = 2/3(a-b)+b = 2/3a+ 1/3b

What about this kind of problem? If you remember that the vector form of this model is

C=( 1-t)a+tb can be answered quickly.

Where t is the proportion of the line segment next to A on the opposite side to the whole opposite side.

This is the conclusion in the book, so it is not a special method, but it has a certain memory component. This is the case with mathematics. Although we want to get to the bottom of it, some conclusions that can improve our operation speed should be remembered.