You can't make a 3 sided object with 4 sides. It's no longer a triangle and is a square. It's the definitions themselves. Non-euclidian geometry would allow the angles to equal more than 180°, but not to "add a side".
Incase that is still a mystery to you, that means that the drawing is visually inconsistent with the finished part. That 180 degree ‘corner’ is effectively a straight line, if drawn to scale.
Technically you are “right” but you are also being obtuse (pun intended).
If you could bare to stretch your mind, and realize “not to scale” means “trust the numbers, not the graphic” you could in turn, realize that it is, in fact, 3 sided.
Every side of every shape is made up of infinite 180 degree angles and 2 angles that are different. Every. Single. One.
My school used to put a right angled triangle in the test and then say “not to scale” and then the angles would actually make it an isosceles or whatever. I agree with you that scale should keep proportions but yeah for some reason It’s not common knowledge.
Mathematicians dont want to to know this one strange trick, hide a secret corner of 180 degrees in any triangle to instantly make it a quadrilateral, without even changing the shape!!
