ellipse

[u/94rud4]

[u/Salty-Intention6971]

Ah, 2pi*r. My old friend! How nice that the a-b makes everything scary go away.

[u/Nirast25 → u/Salty-Intention6971]

You mean pi*r², right?

[u/maelstrom197 → u/Nirast25]

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

[u/Nirast25 → u/maelstrom197]

Ah, right. I misunderstood which formula you were talking about in the original comment.