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Question

The angle made by a double ordinate of length 8a at the vertex of the parabola y2=4ax is


A

π3

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B

π2

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C

π4

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D

π6

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Solution

The correct option is B

π2


Let PQ be a double ordinate of length 8a.
Then PR = RQ = 4a.
Coordinates of P and Q are (OR, 4a) and (OR, -4a) respectively.
Since P lies on the parabola y2=4ax, therefore (4a)2=4a(OR)OR=4a.

Thus, the coordinates of P and Q are (4a, 4a) and (4a, -4a) respectively.
Now, m1=Slope of OP=4a04a0=1and m2=Slope of OQ=4a04a0=1
Clearly, m1m2=1
Thus, PQ makes a right angle at the vertex of the parabola.


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