If `vec a,vec b, vec c` be three vectors such that `vec a+vec b+vec c=0` ,`|veca|=3`, `|vecb|=5` and `|vecc|=7` find the angle between the vectors `veca` and `vec b`.

Explanation

Given , `veca+vecb+vecc=vec0`
`veca+vecb=-vecc`
`(veca+vecb).(veca+vecb)=(-vecc).(-vecc)`
`or a^(2)+b^(2)+2(veca.vecb)=c^(2)`
` or 9+25+2(veca.vecb) =49`
`or veca.vecb=15//2`
`or ab costheta=15//2or 3.5 costheta= 15//2`
`or costheta=1//2 Rightarrowtheta=pi//3`