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
Reflect B(7,2) in the line 2x+y−6=0 to get the point B′ on the incident ray, since the incident ray, its point of incidence, and the image of any point on the reflected ray are collinear.
B′=B−22+122(2x0+y0−6)(2,1),2(7)+2−6=10
B′=(7,2)−520(2,1)=(7−8,2−4)=(−1,−2)
Incident ray through A(3,10) and B′(−1,−2): slope =3−(−1)10−(−2)=3.