JEE AdvancedMathematicsParabolaNumerical+4 / −1
Suppose that the foci of the ellipse are and where and . Let and be two parabolas with a common vertex at and with foci at and , respectively. Let be a tangent to which passes through and be a tangent to which passes through . If is the slope of and is the slope of , then the value of is
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Correct answer: 4
- Find the foci of the ellipse
The ellipse is So, Hence, Therefore the foci are at .
Given: So,
- Write equations of the two parabolas
Parabola
It has vertex at and focus at . A parabola with vertex at origin and focus has equation Here , so
Parabola
It has vertex at and focus at . A parabola with vertex at origin and focus has equation Here , so
- Find slope of tangent to passing through
For parabola A tangent with slope is
For , we have . So tangent is
Since it passes through , Multiply by : Thus,
- Find slope of tangent to passing through
For parabola A tangent with slope is
For , we have . So tangent is
Since it passes through , Multiply by : Thus,
- Compute the required value
We need Substitute: Therefore,
- Comparison with stored answer
Derived answer = . Stored correct answer = . They agree.
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