1.

A double ordinate of the curve y2 = 4ax is of length 8a prove that the lines from the vertex to its ends are at right angles. 

Answer»

Let P = (at2, 2at) and P' = (at2, –2at) be the ends of double cordinate PP'. 

Then

8a = PP' = √(0 + (4at)2) = 4at ⇒ t = 2. 

∴ P = (4a, 4a), P' = (4a, –4a) 

Slope of bar(AP) × slope of bar(AP')

(4a/4a)(- 4a/4a) = –1 

∴ ∠PAP' = π/2.



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