Consider the region $R=\left\{(x, y) \in \mathbb{R} \times \mathbb{R}: x \geq 0\right.$ and $\left.y^{2}…

Consider the region $R=\left\{(x, y) \in \mathbb{R} \times \mathbb{R}: x \geq 0\right.$ and $\left.y^{2} \leq 4-x\right\}$. Let $\mathcal{F}$ be the family of all circles that are contained in $R$ and have centers on the $x$-axis. Let $C$ be the circle that has largest radius among the circles in $\mathcal{F}$. Let $(\alpha, \beta)$ be a point where the circle $C$ meets the curve $y^{2}=4-x$.
The radius of the circles C is____.

Solution

equation of normal at P is y-β=2βx-4+β2

Passing through (r,0)

-1=2r-4+β2 ...i

or   β=0 ...ii

Also r2=r+β2-42+β2 ...iii

from i and iii

r=32,-52

so radius of circle is 1.5

and α=2

from ii and iii

r=2 but in this case circle intersect at three points

Asked in: JEE Advanced 2021 (Paper 2)

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