What's the difference between vector a and vector / A / / AB / and / A / / B /

What's the difference between vector a and vector / A / / AB / and / A / / B /

Vector a is a vector with direction and size. This is wrong. It should be vector a, which means the size of vector AB is the absolute value of the product of vector a and vector B. for example, a = (1,2), B = (2, - 1), then AB = 1 * 2 + 2 * (- 1) [* denotes a multiplier] = 0, so AB = | 0 | = 0 | a | B |