The projection of a vector in the direction of b vector is 2/3→ let the included angle between the two vectors be θ. Then |a|*cosθ=2/3, ∴a*b=|a||b|*cosθ=2. Choose C.

The second question: To calculate the included angle of vectors, we must ensure that the starting points of the two vectors are the same.

Move AB vector and BC vector to the same starting point (point B). A vector *b vector > 0, indicating that the angle between the two vectors must be acute. So the angle ABC is obtuse (the two angles are complementary). Choose C.

The third problem is the coordinate operation of vectors. AB vector coordinate =B point coordinate -A point left. When calculating the length, |a|= in the root sign (x? +y？ ), and then use the product of the band number to calculate the included angle, which is also possible, but not the specific one, as far as the answers of the first two are concerned. Looks like C.