Let the values of p, for which the shortest distance between the lines x+13= y4= z5 and r=(p i+2 j+ k)+ (2 i+3 j+4 k) is 1 6, be a, b, (a<b). Then the length of the latus rectum of the ellipse x2a2+ y2b2=1 is :
- 32
- 9
- 18
- 23
Answer
(4) 23
Solution
Line 1: through (-1, 0, 0), direction b1=3 i+4 j+5 k
Line 2: through (p, 2, 1), direction b2=2 i+3 j+4 k
b1 b2=(16-15) i-(12-10) j+(9-8) k= i-2 j+ k, | b1 b2|= 6
a2- a1=(p+1) i+2 j+ k
S.D. = |(p+1)-4+1| 6= |p-2| 6= 1 6
p=1 or p=3 a=1, b=3
Ellipse x21+ y29=1: major axis along the y-axis (semi-major axis 3, semi-minor axis 1)
Latus rectum = 2 (semi-minor)2semi-major= 2 13= 23
Answer: (4)