Question
Question: The normals at \(\left( x_{1},y_{1} \right),\left( x_{2},y_{2} \right)\) on the parabola \(y^{2} = ...
The normals at (x1,y1),(x2,y2) on the parabola
y2=12x, meet again on the parabola, then x1x2 + y1y2=
A
147
B
108
C
27
D
–27
Answer
108
Explanation
Solution
If the normals at t1 and t2 intersect on the parabola, then t1t2 = 2.
∴ x1x2 + y1y2 = at12.at22+2at1.2at2
= a2(t1.t2)2 + 4a2(t1.t2)
= 12a2 = 108