... IV:m(a, b)=(ma, mb),m∈ R.Thˆa.tvˆa.yt`u.IV v`a III ta c´om(a, b)=(m, 0 )( a, b)=(ma − 0 · b, mb +0·a)=(ma, mb).Nˆe´u m ∈ N th`ı theo II ta c´o(a, b)+(a, b) =(2 a, 2b); (2 a, 2b)+(a, b) =(3 a, ... l`a c˘a.p (1 , 0) v`ır˘a`ng (a, b )( 1 , 0) = (a · 1 − b · 0,a· 0+b · 1) = (a, b); v`a phˆa`ntu.’-khˆong cu’a C l`a c˘a.p(0, 0) v`ı r˘a`ng (a, b) +(0 , 0) = (a +0,b+ 0)= (a, b).Dˆe’ch´u.ng ... nhu.nh˜u.ngc˘a.p(a, 0) v`a (c, 0) l`a b˘a`ng c˘a.p(a + c, 0+ 0)= (a + c, 0). Nhu.ng theo tiˆen dˆe`IV th`ı (a + c, 0) ≡ a + c.Nhu.vˆa.y(a, 0)+ (c, 0)= (a + c, 0+ 0)= (a + c, 0) ≡ a + ct´u.cl`adˆo`ng...