How to extend line off side of shape?

Question

Is there an easy way to extend lines off the side of the shape, with perfectly the same angle as the edge of the shape. Without having to zoom in as far as possible and just eye it. I feel like there has to be an easy way to do this, but I can't figure it out.

Answer 1

1. grab the white Arrow (A) and click on the segment you wish to extend

2. Make a copy of it, Ctrl+C, and paste it in front, Ctrl+F

3. extend the path on both ends using the Scale Dialog

Answer 2

I don't believe there's a tool to do that specifically, but I think the solution itself is quite simple.

For these kind of things, I use guides, the rotate tool and the Pen tool.

1. Create a guide and have it cross the top anchor point of that corner.
2. With the rotate tool selected, set the rotation anchor to the anchor point (hint: with View > Snap to Point activated it´s easy-peasy to snap the rotation anchor).
3. Rotate the guide to match the line you are aiming for.
4. Do the same with a new guide for the lower anchor point of the two.
5. Having done this, you'll two guides perfectly aligned with your shape, similar to your red lines. Now, with the Pen tool, create a shape that matches the new portion, and with the Pathfinder join both shapes (the original and the new one created from the guides).
6. Clean up: remove unnecessary anchor points and consider having a copy saved of both the before and after states, for future uses.