1. If there exists an n × n matrix A−1 such that AA−1 = In = A−1A, then A−1 is the ________ of A.
2. A matrix that has an inverse is invertible or ________. A matrix that does not have an inverse is ________.
3. A 2 × 2 matrix is invertible if and only if its ________ is not zero.
4. If A is an invertible matrix, then the system of linear equations represented by AX = B has a unique solution given by X = ________.