If  A0,4,1, B2,3,−1,C4,5,0respectively, then the angle between AB→ , BC→ is  α2where α=

The direction ratios of the line joining the points Aand B is2,−1,−2The direction ratios of the line joining the points Band C is2,2,1If θ is the acute angle between two lines having direction ratios a1,b1,c1and a2,b2,c2then cosθ=a1a2+b1b2+c1c2a12+b12+c12⋅a22+b22+c22Given that the angle between the lines AB→ , BC→ isα2It implies thatcosα2=4−2−299=0Hence, α2=π2⇒α=π