Take the 2-minute tour ×
Mathematics Stack Exchange is a question and answer site for people studying math at any level and professionals in related fields. It's 100% free, no registration required.

from the figure below enter image description here

there is given that length of BC=3 we are asked to find scalar product of vectors AB and BC i dont know methods to solve such problems and please help me

share|improve this question
    
I can't parse this. Where's your origin located? –  J. M. Aug 16 '11 at 14:11
    
aa ok see this it is written in georgian language but question is the same as i wrote see it naec.ge/images/doc/MASC_SERT/2011pedagogebi-matematikai.pdf number 19 –  dato datuashvili Aug 16 '11 at 14:18
add comment

1 Answer

up vote 1 down vote accepted

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 $\vec{BA}\cdot\vec{BC}=-|\vec{BC}|^2$.

share|improve this answer
    
how this equality does hold?BA*BC=|BC||AB|cos(∠ABC) –  dato datuashvili Aug 16 '11 at 14:40
    
sorry i understood –  dato datuashvili Aug 16 '11 at 14:41
    
thanks very much @robjohn i have seen everything and got right answer you answer helped very much thanks ones again –  dato datuashvili Aug 16 '11 at 14:49
add comment

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.