Key idea Matrix addition/subtraction is entry-wise: (A±B)ij=aij±bij(A\pm B)_{ij}=a_{ij}\pm b_{ij}(A±B)ij=aij±bij Parentheses matter in grouped expressions like (A−B)−C(A-B)-C(A−B)−C and A−(B−C)A-(B-C)A−(B−C).