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
We have a well known inequality that for any point $O$ interior to a triangle,
$OA + OB + OC \gt s$ where $s$ is sub-perimeter.
Here, $OA = OB = OC = R$ (circumradius)
So, $R \gt \dfrac43$ and hence $R + r \gt \dfrac 43$
As the circumcenter $O$ is inside the triangle, we have an acute triangle a...