Question

If $\vec{a}, \vec{b}, \vec{c}$ are such that $\vec{a}+\vec{b}+\vec{c}=0 \ and \ |\vec{a}|=3,|\vec{b}|=5 \ and \ |\vec{c}|=7$, then find angle between $\vec{a}\ and \ \vec{b}$

Answer 1

a+b+c=0

a+b=-c

(a+b) ^2 =c^2

a^2 +b^2 +2(a.b)=c^2

a^2 +b^2 +2(a)(b)cos (theta) =c^2

9+25+2*3*5 cos(theta) =49

Cos(theta) =1/2

Theta =60°