Problem 13 - Entrance Test

If A = [[1, 2], [3, 4]], then adj A is:

A. [[-4, 2], [3, -1]]B. [[-4, -2], [-3, 1]]C. [[-4, -3], [-2, 1]]D. [[4, -2], [-3, 1]]

Correct: D

For a 2x2 matrix A = [[a, b], [c, d]], the adjugate (adj A) is given by [[d, -b], [-c, a]]. Here, A = [[1, 2], [3, 4]], so adj A = [[4, -2], [-3, 1]].