# Parabolic arc

**URL:** <https://discourse.shapr3d.com/t/parabolic-arc/14002>\
**Category:** Modeling Questions\
**Created:** [July 22, 2021, 7:41pm UTC](https://discourse.shapr3d.com/t/parabolic-arc/14002 "2021-07-22T19:41:49Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![Alessio](https://avatars.discourse-cdn.com/v4/letter/a/d6d6ee/32.png) [@Alessio](https://discourse.shapr3d.com/u/Alessio)\
**Post date:** [July 22, 2021, 7:41pm UTC](https://discourse.shapr3d.com/t/parabolic-arc/14002/1 "2021-07-22T19:41:50Z")

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Hi  
There is a way to draw a 2D parabolic arc, not a spline, mathematically defined?  
I’m a structural engineer and some arc bridge had to have a parabolic shape for best behaviour  
And I know I can draw it in other Cad software and import the file 🤣

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**Author:** ![PEC](https://avatars.discourse-cdn.com/v4/letter/p/67e7ee/32.png) [@PEC](https://discourse.shapr3d.com/u/PEC)\
**Post date:** [March 4, 2022, 6:22pm UTC](https://discourse.shapr3d.com/t/parabolic-arc/14002/2 "2022-03-04T18:22:41Z")

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Hi,  
it’s quite an old post, but here is a fast and accurate solution to draw a parabolic arc.

3 points Shapr3D’s spline in **control mode** is actually an arc of parabole when the first point is the origin of the parabole, the third point is the end of the arc of parabole and the mid point is well positioned on the (Ox) axis at mid distance.  
Here is a short video that is self explanatory.

  
I added a quick complement to draw both branches of the parabole using only 3 points spline in **control mode** as well as finding Focus point and Directrix line and a quick illustration of one of the nice properties of parabolic curves : every light ray arriving “vertically” is reflected to the focus point.