let K be Ring; :: thesis: for M1, M2, M3 being Matrix of K st - M1 = - M2 holds
M1 = M2
let M1, M2, M3 be Matrix of K; :: thesis: ( - M1 = - M2 implies M1 = M2 )
assume - M1 = - M2 ; :: thesis: M1 = M2
then - (- M1) = M2 by Th1;
hence M1 = M2 by Th1; :: thesis: verum