Update index.html

Author: Gmac4247

index.html

@@ -2003,7 +2003,7 @@ <h3 itemprop="name" style="margin:7px">The Area of a Circle</h3>
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-<section itemprop="description" itemref="trigonometry" id="circle_area_proof">
+<section itemprop="description" id="circle_area_proof">
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   <summary><h4 style="margin:7px">When the overlapping area equals to the uncovered area in the middle, the sum of the areas of the quadrants is equal to the area of the square. That square represents the area of the circle in square units.
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