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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)\left( x_{1},y_{1} \right),\left( x_{2},y_{2} \right) on the parabola

y2=12xy^{2} = 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+2at1.2at2at_{1}^{2}.at_{2}^{2} + 2at_{1}.2at_{2}

= a2(t1.t2)2 + 4a2(t1.t2)

= 12a2 = 108