# If a+b+c=0 and |a|=3,|b|=5,|c|=7, then angle between vectors a and b is??

Ashik Raveendran
11 Points
4 years ago
$a + b +c =0$
then, $a + b =-c$
taking modulus on both sides ,$\left |a+b \left | = \left | -c \right |$
$\Rightarrow \left | a+b \right |^{2}=\left | -c^{} \right |^{2}$
$a^{2}+2\vec{a}\vec{b}+b^{2}=c^{2}$
$abcos\theta =c^{2}-a^{2}-b^{2}$
$\Rightarrow cos\theta =c^{2}-a^{2}-b^{2}/2ab$
substituting the values we get ,$cos\theta =1/2$
$\Rightarrow \theta =60^{o}$
All the bes