Question 259789
Consider one quadrant:
x^2 + y^2 = r^2
y = sqrt(r^2 - x^2)
Area = x*y
Area = x*sqrt(r^2 - x^2)
Total area = 4xsqrt(r^2 - x^2)
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To use x as one side, not 1/2 of one side:
Area = 2x*sqrt(4r^2 - x^2)