Let Q and R be the feet of perpendiculars from the point P(a, a, a) on the lines x=y, z=1 and x=-y, z=-1 respectively. If is a right angle, then 12a2 is equal to .
Answer
12
Solution
Line 1: (r, r, 1), d.r. (1, 1, 0); Line 2: (k, -k, -1), d.r. (1, -1, 0)
PQ line 1: (r-a)+(r-a)=0 ⇒ r=a ⇒ Q(a, a, 1)
PR line 2: (k-a)-(-k-a)=0 ⇒ k=0 ⇒ R(0, 0, -1)
PQ=(0, 0, 1-a), PR=(-a, -a, -1-a)
QPR=90 : PQ PR=-(1-a)(1+a)=0 ⇒ a2=1
12a2=12
Answer: 12