Find the radius of circle, given tangents from three vertices of a square

circleseuclidean-geometrygeometryplane-geometry

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The above is the image of the question. The brown, blue, green lines are tangent to the circle and the measure of a side of the square is 10. The question is to find the value of the radius.

My Attempt:

I took O as the centre of the circle and drew radius to the tangent point of each tangent. Then I found the lengths of the lines joining the circumcenter and each vertex of the square. But I didn't find any useful relations among the attempts I tried.

So anyone in this community could help me with this question.

Sorry for the bad writing in the picture.

Thank you!

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Best Answer

HINT...let the coordinates of the centre of the circle be $(x,y)$ with the bottom left-hand corner of the square being the origin. Then, considering distances from the origin and from other corners of the square to the centre of the circle, you have: $$x^2+y^2=64+r^2$$ $$x^2+(10-y)^2=9+r^2$$ and $$(10-x)^2+(10-y)^2=49+r^2$$

You can solve these simultaneously to get $x$ and $y$ and hence get $r$.