How to draw a circle circumscribed and tanger to other two


Hi everybody.

Not sure if I’m going to use the right English words to describe what I’m looking for.
My intention is to draw a circle circumscribed and tangent to other two.


Let’s suppose I have a circle called A and another one called B.
I’d like to know how I can find

  1. the center of the arc I drew in “red/pink”
  2. the tangent points of the big circle on A and B.

I know TIG developed an awesome extension called True Tangents, not sure if it can help in this situation but I wasn’t able to use it to solve this situation.

I know little maths and nothing about architecture, but all kind of help will be appreciate.

Thank you so much.



In theory, you would need one extra parameter ( diameter circle ) , for there are an infinite number of possibilities…!

Edit : Oops, did not see @DaveR already applied, btw: is there an ejercicio number :slight_smile:

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Could Discourse show who is typing an answer to a specific topic?

Hi Mike, thank you.

I was trying this exercise. I thought it wasn’t possible to deduct the diameter.
Any idea is welcome

EDIT: I don’t see that info on the blueprint.

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I didn’t pay attention well to the blue print.
The radio is 118.

And ecati told me how I can face the situation Can I draw an arc tangent to two circles?

TIG also said it was possible with his extension but I didn’t find the way with the extension, but I did with ecati method.

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@MikeWayzovski, I didn’t understand your quote “btw: is there an ejercicio number”, are you asking me if it’s an exercise?



You ‘Oneboxed’ the exercise (ejercicio 14) in your reply



I quoted that site because my question had to do with the proposed exercise.
I’m still not sure if I understood what you meant.



You have quoted that site a few times, lately, and so I presumed that there was a relation to that site.



@MikeWayzovski I prefer you to make clear the question is on your mind and it will be a pleasure for me to answer it.



The red arc can be any radius greater than half of the sum of the distance between the centers of A and B, and Arad+ Brad
The minimum value is a circle which is tangential with A and B and is centered on a line drawn from A to B, offset appropriately.
The maximum radius [infinity] results in an arc than is effectively a straight line tangentially to A and B.
See the image for the need for an Xrad for an A & B tangential arc, and finding it’s center.



Awesome explanation TIG, I understood that I need to know the diameter or the center.
The radius is on the blueprint, but I didn’t realized at first.
Thank you.


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