I am using Adobe Illustrator and Adobe After Effects. I want to animate an SVG triangle. I want the Hypotenuse to "swing out" from the bottom leg (go from 0 deg to 45deg). I can do this by 1), making each triangle segment its own layer, 2) setting an anchor point at the bottom left vertex (see the graph), and then 3), going into after effects and change the rotation of the hypotenuse line segment. The problem is that the line segments do not join properly. As you can see the vertices are not filled in. I can fix this by joining the the segments, but then all the segments become one layer, and I cannot animate the hypotenuse swinging.
Round line ends fit in different angles:
If you have a closed triangle you can move one of its anchor points. The lines follow. Round corners should be used for non-peaky look.
Grey and green triangles were originally equal. The top corner anchor point of the green triangle is moved upwards.
Scaling (=dragging the top side of the bounding box) does the same. Prevent Stroke width scaling in the preferences!
ADD due a comment:
You can well have a closed triangle. Shrink it vertically from full height to a line and then horizontally from full width to a single point. Reverse the animation frame sequence to get a video which presents the growth from a single point to the full triangle.