A1: ( width M1 = n & width M2 = n & len M1 = n & len M2 = n ) by MATRIX_1:25;
then ( len (M1 + M2) = len M1 & width (M1 + M2) = width M2 ) by MATRIX_3:def 3;
hence M1 + M2 is Matrix of n,K by A1, MATRIX_2:7; :: thesis: verum