One nitpick: Part I says "Thanks to the fact that we are in an abelian group, we can write P + Q + R = 0 as P + Q = –R" I'm pretty sure abelianness is not necessary. P + Q + R = 0 => P + Q + R + -R = 0 + -R => P + Q + 0 = -R => P + Q = -R
As long as we're nitpicking, Taking the definition of cofator just ain't right.
One nitpick: Part I says "Thanks to the fact that we are in an abelian group, we can write P + Q + R = 0 as P + Q = –R" I'm pretty sure abelianness is not necessary. P + Q + R = 0 => P + Q + R + -R = 0 + -R => P + Q + 0 = -R => P + Q = -R