Set of values of ‘h’ for which the number of distinct common normals of
(x – 2) 2 = 4(y – 3) and x 2 + y 2 – 2x – hy – c = 0 (c > 0) is 3, is
Text Solution
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The equation of any normal
(x – 2) 2 = 4(y – 3) is x – 2 = m (y – 3) – 2m – m 3 If it passes through (1, h/2), then
1 – 2 = m
– 2m – m
3 ⇒ 2m 3 + m
(10 – h) – 2 = 0
This equation will give three distinct values of m, if ƒ ′ (m)
= 0 has two distinct roots,
where ƒ(m) = 2m 3 + m (10 – h) – 2
Now ƒ ′ (m) = 6m 2 + (10 – h)
ƒ ′ (m) = 0 ⇒ m ± 
The values of m are real and distinct if h > 10 i.e. h ∈ (10, ∞ ).
Hence is correct answer.
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