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Question: Answered & Verified by Expert
Let $A B C D$ be a square of side length 1 , and $\Gamma$ a circle passing through $B$ and $C$, and touching $\mathrm{AD}$. The radius of $\Gamma$ is
MathematicsCircleKVPYKVPY 2015 (SA)
Options:
  • A $\frac{3}{8}$
  • B $\frac{1}{2}$
  • C $\frac{1}{\sqrt{2}}$
  • D $\frac{5}{8}$
Solution:
2287 Upvotes Verified Answer
The correct answer is: $\frac{5}{8}$


$\mathrm{PC}=\mathrm{r}$
$\mathrm{PC}^{2}=\mathrm{r}^{2}$
$(\mathrm{r}-1)^{2}+\left(\frac{1}{2}-1\right)^{2}=r^{2}$
$1-2 \mathrm{r}+\frac{1}{4}=0$
$r=\frac{5}{8}$

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