Given ABCD is any four points on the plane, then vector AB + vector CD + vector DA =?

Given ABCD is any four points on the plane, then vector AB + vector CD + vector DA =?

Vector CB

Given the absolute value of vector a=3, vector b=(1,2), and vector a is perpendicular to vector b, the coordinate of vector a is Given the absolute value of vector a=3, vector b=(1,2), and vector a is perpendicular to vector b, then the coordinate of vector a is

Let a=(x, y),|a|=3, i.e. x^2+y^2=9
A is perpendicular to b, then x*1+y*2=0, i.e. x=-2y
Y^2+4y^2=9
Y^2=9/5
Y =(+/-)3/5 No.5
I.e. a=(-6/5 root 5,3/5 root 5) or (6/5 root 5,-3/5 root 5)

Why is a zero vector equal to a parallel vector, but the definition of a parallel vector states that non-zero vectors are parallel? Uh... Better to explain it a little more clearly,

A zero vector is an equal vector as a special case. Just remember it alone. It's not necessary. It's also necessary for other related knowledge.

Is the zero vector collinear with the zero vector By proving that B =λA (if A is not equal to the zero vector, then how can we know that the zero vector is collinear with the zero vector

0 Vector is collinear with any vector
This is in addition to the collinear definition

If the length of vector a is 1 and vector b is equal to -5a in space, the product of vector a and vector b

A*b=[ a ]*[ b ]* COS180=1*5*[-1]=-5

If the module of vector a=3, the module of vector b=5, and vector ab=12, then the projection of vector a in the direction of vector b is

12/5