Let a ray of light passing through the point (3, 10) reflects on the line 2x + y = 6 and the reflected ray passes through the point (7, 2). If the equation of the incident ray is ax + by + 1 = 0, then a2 + b2 + 3ab is equal to .
Answer
1
Solution
∵ Incident ray produced passes through B', the image of B(7, 2) in the mirror line
x - 72 = y - 21 = -2(14 + 2 - 6)5 = -4 B'(-1, -2)
[figure]
Incident ray = line AB': slope = 10 + 23 + 1 = 3
y - 10 = 3(x - 3) 3x - y + 1 = 0 a = 3, b = -1
∴ a2 + b2 + 3ab = 9 + 1 - 9 = 1
Answer: 1