Question

DOUBLE ORD
16. Number of focal chords of the parabola
y? = 9x whose length is less than 9 is
1) 2
2) 5 3) 1 4) 0
2
ar nearest to the focus​

Answer 1

Given : parabola y² = 9x

To find : Number of focal chords of the parabola whose length is less than 9

Solution:

Length of focal chord

for y² = 4ax

is given by 4a.cosec²α

α is the angle with x axis

Comparing y² = 4ax  and y² = 9x

Hence 4a = 9

Length of focal chord = 9 Cosec²α

Cosec²α ≥ 1

Length of focal chord ≥ 9

Number of focal chords of the parabola y² = 9x whose length is less than 9 is ZERO (0)

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