If the image of the point P(a, 2, a) in the line x2 = y + a1 = z1 is Q and the image of Q in the line x - 2b2 = y - a1 = z + 2b-5 is P , then a + b is equal to .
Answer
3
Solution
Q is the image of P in L1 and P is the image of Q in L2, so the midpoint M of PQ lies on both lines and PQ L1, PQ L2.
M is the point of intersection of L1 and L2.
L1: (2s, s - a, s); L2: (2b + 2u, a + u, -2b - 5u)
Equating: 2s = 2b + 2u, s - a = a + u, s = -2b - 5u
s = b + u = 2a + u b = 2a
b + u = -2b - 5u u = - b2, s = b2 = a
M = (2a, 0, a)
MP = (a - 2a, 2 - 0, a - a) = (-a, 2, 0) must be to L1 (d.r. 2, 1, 1):
-2a + 2 + 0 = 0 a = 1
Check with L2 (d.r. 2, 1, −5): -2 + 2 + 0 = 0 ✓
b = 2a = 2
a + b = 1 + 2 = 3
Answer: 3