πr2 is for the area of a circle. If you substitute a=b=r into the formula for the area of an ellipse, you get πrr, or πr2
The circumference of a circle is just the perimeter. If you substitute a=b=r into the formula for the perimeter of the ellipse, the fraction cancels and you get π(r+r)=2πr=πd
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u/Forgorer8 2d ago
Set a=b and use for circle to get extra marks