The cosine of two vectors

Question

For calculating a cosine one needs a right-angled triangle.
So is the cosine of two vectors the cosine of any right-angled triangle that you can "build" in some way from these vectors ?

triangle created with two vectors

Answer 1

I assume it refers to the cosine of the angle between these two vectors. It is closely related to the inner product of these two vectors: $$ \vec{a}\cdot \vec{b}=|\vec{a}|\cdot|\vec{b}|\cdot \cos \sphericalangle (\vec{a},\vec{b}) $$