Comprehension

Consider the region R = {(x,y)∈ R×R : x ≥ 0 and y2 ≤ 4 – x. Let F be the family of all circles that are contained in R and have centres on the x-axis. Let C be the circle that has the largest radius among the circles in F. Let (α, β) be a point where circle C meets the curve y2 = 4 – x.

Question: 1

The radius of the circle 𝐶 is ___ .

Updated On: May 23, 2024
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Correct Answer: 1.5

Solution and Explanation

Let the circle be,

(x – a)2 + y2 = r2
Solving it with parabola
y2 = 4 – x we get
(x – a)2 + 4 – x = r2
x2 – x(2a + 1) + (a2 + 4 – r2) = 0 …(1)

D = 0
\(⇒\) 4r2 + 4a – 15 = 0
Clearly a ≥ r
So 4r2 + 4r – 15 ≤ 0
\(⇒\) rmax =\(\frac{3}{2}\) = a
Radius of circle C is \(\frac{3}{2}\)

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Question: 2

The value of 𝛼 is ___ .

Updated On: May 23, 2024
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Correct Answer: 2

Solution and Explanation

Let the circle be,
(x – a)2 + y2 = r2
Solving it with parabola
y2 = 4 – x we get
(x – a)2 + 4 – x = r2
x2 – x(2a + 1) + (a2 + 4 – r2) = 0 …(1)
D = 0

\(⇒\) 4r2 + 4a – 15 = 0
Clearly a ≥ r
So 4r2 + 4r – 15 ≤ 0
\(⇒\) rmax\(\frac{3}{2}\) = a

Radius of circle C is \(\frac{3}{2}\)

From (1) x2 – 4x + 4 = 0

\(⇒ x = 2 = α\)

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