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What I did:

r, radius of circle

d, diameter of circle

NOTE: diameter is also diagonal of square, therefore base of a triangle that is half of the square...

 

 

 

 

 

If you split the square into 2 triangles, they each have a height of r and a base of 2r...so...

Atri = 1/2(r)(2r) = r2

Asquare = 2(r2) = 2r2

We know the area of the circle must be 3.14r2

Now...to find the area of the circle outside the square we do: 3.14r2 - 2r2 = 1.14r2

To find the percent of the original circle that is, I think we do: 1.14r2  = .3630573
                                                                                                   3.14r2

That rounds to about 36.3%, or 36% I guess...I think this should work...she explained the triangles thing...