Roberts's triangle theorem
... that it took 90 years to replace the "unconvincing" original proof of Roberts's triangle theorem, on the number of triangles formed by systems of lines, with a correct proof?
... that it took 90 years to replace the "unconvincing" original proof of <b id="mwBQ4"><a href="./Roberts's_triangle_theorem" id="mwBQ8" rel="mw:WikiLink" title="Roberts's triangle theorem">Roberts's triangle theorem</a></b>, on the number of triangles formed by systems of lines, with a correct proof?