Hi, I would like to draw lines of defined slope so I set an angle with the measuring tool but when I change tool (the line or the brush) the measuring tool disappears.
Is there a way to prevent it from disappearing?
Thank in advance

The measuring tool is for measuring; what you want is the assistant tool.

Which of the assistants? I have only the ellipse, the perspective, the vanishing points, the rulers and the spline. What I need is a tool that allows me to see what angle my line has.

You can use the Measure tool as a guide to construct a line at a particular angle.
Set the letter 'M' to be a keyboard shortcut for the Measure tool (or whatever letter is free that you prefer).
Set your brush preset, colour and size to what you want. Use a mouse to avoid hand/stylus shake.

Using Brush, place the cursor at the start of the line you want to construct and do a click to make a start dot.
Press 'M' to switch to the Measure tool then click-hold-drag a measuring line out to the distance and angle you require. Then release the click and press the 'B' key for the brush. Press and hold the 'V' key to make the Line tool active. Click-hold-drag the cursor back to the start dot that you made before and release the click to draw the line.

It works.
Thank you very much!