When I use the tool: tangent to two entities, through a point, and the point belongs to one of the two entities, Qcad does not solve the problem.

Works well if there are two lines,

but if it is a line and a circle gives false solutions,

and if it is two circles say that there is not solutions.

Tested in Debian Gnu/linux 10 and Qcad 3.26.4.0

## tangent to two entities, through a point, when the point belongs to one of the two entities

**Moderator:** andrew

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### Re: tangent to two entities, through a point, when the point belongs to one of the two entities

Sorry, never heard about a tool called "Tangent to two entities". I know about a tool called "Tangent two circle (LT2) which works between two circle. Is that the tool what you refer to?

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### Re: tangent to two entities, through a point, when the point belongs to one of the two entities

mangel: please attach your drawing file with the two tangential entities and indicate which exact tool you are using, thanks.

### Re: tangent to two entities, through a point, when the point belongs to one of the two entities

Draw circle tangential to two entities, through a point (CT2).

Andrew, all my findings are in the attached dxf files ... Not limiting.

- With 2 lines and a point on one of the lines QCAD presents 2 correct solutions.

The indicated point is then the (forced) tangent point with that line.

Solutions reduce to one unique if the lines are parallel.

- With a circle, a line and a point on the line QCAD may present 4 solutions of which none is correct.

If the indicated point is exactly the center projected perpendicular on the line, a lucky shot or by an auxiliary line, ...

... then 2 correct solutions are presented.

With 4 solutions it is clearly visible that the indicated point is an intersection with the line and that all the solutions intersect twice. For the correct solutions:

The tangent point of two circles is on a line through both centers. (Cyan)

And the center of a circle tangent to a line is on a line orthogonal in the tangent point. (Blue)

One can find these correct solutions by the intersection of 2 incorrect solutions.

- Analog with a line, a circle and a point on the circle.

- With two circles there will not be a solution for a point inside one of the circles.

The solution can't be and tangent to and crossing the circle at the same time.

For a point outside the circles 4 solutions may be presented. Not all 4 are correct in all cases. For a point on one of the circles there are

**no**solutions presented.

These can be found by mirroring the other circle over the symmetry axis center - point and matching solutions of 3 tangents (CT3).

This knowledge is gathered in the past few months while I investigated how to round all corners of bulging polylines.

It boils down to what is considered tangent, what is the intersection of two circles, and how arc segments are prepended/appended to a polyline.

The tool is perfectly functional up to a point where the numbers don't add up anymore. They should mathematically.

Regards,

CVH