From the figure below
It is given that length of BC=3#
we are asked to find scalar product of vectors AB$ and $BC$
I don't know methods to solve such problems and please help me.
From the figure below
It is given that length of BC=3#
we are asked to find scalar product of vectors AB$ and $BC$
I don't know methods to solve such problems and please help me.
If by scalar product, you mean dot product, then the answer would be $-9$. The definition of cosine says $|\vec{BC}|=|\vec{AB}|\cos(\angle ABC)$; note that $cos(\angle ABC)>0$ since it is acute. Since $\vec{BA}\cdot\vec{BC}=|\vec{BC}||\vec{AB}|\cos(\angle ABC)$ and $\vec{AB}=-\vec{BA}$, we get that $\vec{BA}\cdot\vec{BC}=-|\vec{BC}|^2$.