Let a line L passing through the point (1, 1, 1) be perpendicular to both the vectors 2 i+2 j+ k and i+2 j+2 k. If P(a, b, c) is the foot of perpendicular from the origin on the line L, then the value of 34(a+b+c) is :
- 50
- 80
- 100
- 120
Answer
(3) 100
Solution
Direction of L: (2 i+2 j+ k) ( i+2 j+2 k)= vmatrix i& j& k\\2&2&1\\1&2&2 vmatrix=2 i-3 j+2 k
Any point of L: P(1+2t, 1-3t, 1+2t)
P is the foot from O ⇒ OP L:
2(1+2t)-3(1-3t)+2(1+2t)=0
1+17t=0 t=- 117
a+b+c=(1+2t)+(1-3t)+(1+2t)=3+t= 5017
∴ 34(a+b+c)=100
Answer: (3)