You guys know of any good site that explains fully about how Lambert proved that Saccerei’s acute angles does not result in a contradiction to the fifth postulate of Euclidean geometry? Thank you for any help
Hurkyl, no one said he did. The original poster asked about his "proof" that Saccheri's work was invalid.Lambert proved no such thing. He merely explored the fields of neutral and hyperbolic geometry. He was unable to derive a contradiction in hyperbolic geometry, but he certainly did not prove hyperbolic geometry was contradiction free.