A vector,c→ directed along the internal bi-sector of the angle between the   vectors a→=7i^−4j^−4k^ and b→=−2i^-j^+2k^, with |c→|=56, is

The required vectors c→ is given by λa→a→+b→b→now a→a→=197i^−4j^−4k^ and b→b→=13−2i^-j^+2k^⇒          c→=λ19i^−79j^+29k^⇒c→2=λ2×5481 Therefore, λ2=225 or λ=±15⇒                                                                     c→=±53i^−7j^+2k^