Question Details

Consider the hyperbola x2 100 y2 64 = 1 with foci at  S  and S1 , where    lies on the positive x-axis. Let  P  be a point on the hyperbola, in the first quadrant. Let SPS1 = α , with  α < π 2 . The straight line passing through the point  S  

and having the same slope as that of the tangent at  P  to the hyperbola, intersects the straight line  S1P at P1 . Let δ be the distance of P from the straight line  S P1 , and β = S1P . Then the greatest integer less than or equal

to  β δ 9  sin α  2 is __________.

Show Answer

Correct Answer :

7

Solution :

The correct answer is 7.


Step 1: Understand the given hyperbola and key parameters
The equation of the given hyperbola is:

x2 100 y2 64 = 1

Here, a2 = 100 (so a = 10 ) and b2 = 64 (so b = 8 ).
The eccentricity e of the hyperbola is calculated using:

e = 1 + b2 a2 = 1 + 64 100 = 164 10

The distance from the center to each focus is ae = 164 .
Hence, the foci are located at S = (ae,0) and S1 = (ae,0) .


Step 2: Apply the reflection property of the hyperbola
A fundamental property of hyperbolas states that the tangent at any point P bisects the interior angle between the focal radii PS and PS1.
Thus, the tangent at P is the angle bisector of SPS1 = α .
The straight line passing through focus S parallel to the tangent at P intersects the line S1P at P1.
In triangle P S P1:
Since the line S P1 is parallel to the tangent (which bisects SPS1 ), the line S P1 makes an angle of α2 with both PS and PP1.
Therefore, triangle P S P1 is an isosceles triangle with PP1 = PS .


Step 3: Relate the focal distances and length β
By definition of a hyperbola, for any point P on it:

S1P SP = 2a

Given β = S1P , we have:

PS = β 2a = β 20

Since PP1 = PS , the distance δ from P to the line S P1 is the height of the isosceles triangle P S P1 with equal sides of length PS and apex angle α:

δ = PS cos α2

Also, the base S P1 of triangle P S P1 is given by:

SP1 = 2 PS sin α2


Step 4: Express the area of triangle S1 P S
The area of triangle S1 P S can be written in two ways:
1) Using two sides and the included angle α:

Area = 12 S1P SP sinα = 12 β PS 2 sin α2 cos α2 = β δ sin α2

2) Using standard geometric properties of hyperbola tangents and foci:
The perpendicular distance from the foci to any tangent of a hyperbola satisfies p1p2=b2=64 . Evaluating the area of triangle S1P S yields:

Area = b2 cot α2

Equating the expressions, we find:

βδ sin α2 = b2 = 64


Step 5: Compute the greatest integer value
We need to find the greatest integer less than or equal to:

βδ 9 sin α2 = 649 7.11

Taking the greatest integer function (floor value):

649 = 7

Thus, the final answer is 7.

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