AleksandrM wrote:Sorry ian, but I do not follow. I took a book. I look at it, and I see that if I make any of the 90 degree corners wider, this will change the other corners as well, making them NOT 90 degrees.
The question is: if two opposite angles in a 4-sided shape are 90 degree angles, must the shape be a rectangle?
The answer is no, certainly not. You can draw a non-rectangle with opposite angles of 90 as follows:
-draw an L in the bottom left corner of a page, so that the vertical and horizontal lines forming the L are parallel to the borders of the page;
-draw an upside-down L in the middle of the page, *but rotate it by a few degrees* so that the vertical and horizontal parts of the L are not parallel with the borders of the page;
-from the L in the bottom left, extend as far as you can the horizontal and vertical lines which form the L;
-do the same with the L in the middle of the page;
-these lines will meet, and the intersection points, along with the two corners of the two Ls, form a quadrilateral. The Ls of course are 90 degree angles, but as long as you rotated the L in the middle of the page, the other two angles will not be 90 degrees.
If you know opposite angles in a quadrilateral are both 90, all you can really say is that the remaining two angles sum to 180.
