Vector a+b+c=0

A, b and c form a vector triangle.

You can find that |a|+|b|=|c|

Then a and b are in the same direction, and c is opposite to a and b.

(2)

|a b|=|a|*|b|*|cosθ|=3

|a×b|=|a|*|b|*|sinθ|=4

|a|*|b|=5.

(7)

Calculate vectors AB, AC, AD

Calculate their mixed product

The length of the module is the volume.