If `veca+vecb+vecc =0, |veca|=3, |vecb|=5, |vecc|=7` then the angle between `veca and vecb` is :

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`