From the point (-1, -1), two rays are sent making angles of 45 with the line x+y=0. These rays get reflected from the mirror x+2y=1. If the equations of the reflected rays are ax+by=9 and cx+dy=7, a, b, c, d Z, then the value of ad+bc is .
Answer
7
Solution
[figure]
x+y=0 has inclination 135 ; lines at 45 to it have inclinations 90 and 180
the rays from P(-1, -1) are x=-1 and y=-1
They meet the mirror x+2y=1 at B(-1, 1) and C(3, -1)
Image of P in the mirror: x+11= y+12= -2(-1-2-1)12+22= 85 P' ( 35, 115 )
each reflected ray, produced backwards, passes through P':
Through C and P': slope = 115+1 35-3=- 43 y+1=- 43(x-3) 4x+3y=9
Through B and P': slope = 115-1 35+1= 34 y-1= 34(x+1) -3x+4y=7
a=4, b=3, c=-3, d=4
ad+bc=16-9=7
Answer: 7