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If the 3x – 4y = k touches the hyperbola x25-4y25 = 1 then find the value of k - Mathematics and Statistics

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Question

If the 3x – 4y = k touches the hyperbola `x^2/5 - (4y^2)/5` = 1 then find the value of k

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Solution

We know that y = mx + c will be a tangent to the hyperbola `x^2/"a"^2 - y^2/"b"^2` = 1 if c2 = a2m2 – b

Given hyperbola is `x^2/5 - (4y^2)/5` = 1

i.e. `x^2/5 - y^2/((5/4))` = 1

∴ a2 = 5, b2 = `5/4`

Given tangent is 3x – 4y = k

∴ 4y = 3x –  k i.e. y = `3/4x - "k"/4`

∴ m = `3/4`, c = `-"k"/4`

Applying tangency condition c2 = a2m2 – b2, we get,

`(-"k"/4)^2 = 5(3/4)^2 - 5/4`

∴ `"k"^2/16 = 5(9/16) - 5/4`

= `45/16 - 5/4`

= `25/16`

∴ k2 = 25

∴ k = ±5.

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Conic Sections - Hyperbola
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Chapter 7: Conic Sections - Exercise 7.3 [Page 175]

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