Let(a,b,c,d)beaprojectiveframeinRP2,andletDbealinenotpassingthroughanyofa,b,c,d.ThelineDintersects〈a,b〉and〈c,d〉inpandp′,〈b,c〉and〈a,d〉inqandq′,and〈b,d〉and〈a,c〉in r and r′. Prove that the re is a unique...

Let(a,b,c,d)beaprojectiveframeinRP2,andletDbealinenotpassingthroughanyofa,b,c,d.ThelineDintersects〈a,b〉and〈c,d〉inpandp′,〈b,c〉and〈a,d〉inqandq′,and〈b,d〉and〈a,c〉in r and r′. Prove that the re is a unique in volution mapping p top′,q to q′, and r tor′.