Since the circle is inside the square, quite snugly, half of the square's side is the circle's radius. 9/2=4.5 and circumference is diameter (radius*2) times pi. 4.5 goes back to 9 and pi is multiplied. 9pi (A much simpler way is to see that the diameter is the side of the square.)
Since the circle is inscribed in the square, the side of the square is the diameter of the circle. So to find the circumference use the formula pi*D where D is the diameter
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