ROWS × COLS
Matrix size = rows × columns. Always state in this order.
INNER = OUTER
AB is defined only when cols of A = rows of B. The result size = outer numbers.
ORDER MATTERS
AB ≠ BA in general. Matrix multiplication is NOT commutative.
ROW · COL
Each entry Pij = dot product of row i of A × column j of B.
IDENTITY I
AI = IA = A. Identity has 1s on diagonal, 0s elsewhere.
SQUARE POWER
A² = A·A only works when A is a square matrix (n×n).
MAIN DIAGONAL
Goes from top-left → bottom-right. Identity has 1s here.
REVENUE APP
Batch × Rate = Result. Real-world matrix products combine quantities × values.