A ray of light coming from the point P(1, 2) gets reflected from the point Q on the x-axis and then passes through the point R(4, 3). If the point S(h, k) is such that PQRS is a parallelogram, then hk2 is equal to:
- 60
- 70
- 80
- 90
Answer
(2) 70
Solution
[figure]
The reflected ray QR, produced backwards, passes through the image P'(1, -2) of P in the x-axis.
Line P'R: y + 2 = 53(x - 1); putting y = 0 gives x = 115, so Q ( 115, 0 ).
In parallelogram PQRS the diagonals PR and QS bisect each other, so S + Q = P + R:
h + 115 = 1 + 4 h = 145 and k + 0 = 2 + 3 k = 5
hk2 = 145 25 = 70
Answer: (2)