How to find the ellipse perimeter and equation using this?

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I draw a circle on a paper and cut a sector of it to make a cone with it.

then I draw an ellipse on the cone. then I netted the cone to have its circular sector again.

the ellipse's perimeter was like what you see in the picture (I have lost it so I draw a similar thing in Microsoft Paint).

how to find ellipse's perimeter, semi-major and semi-minor axes using it?

please don't tell me find the perimeter with a tool. I want a way.enter image description here