Ch. 4.5 2 X 2 Matrices, Determinants, and Inverses.

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4.5 2x2 Matrices, Determinants and Inverses
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Ch X 2 Matrices, Determinants, and Inverses

Square Matrix A matrix with the same number of columns as rows

Multiplicative Identity Matrix For an n X n square matrix, the multiplicative identity matrix is an n X n square matrix I, or, with 1’s along the main diagonal and 0’s elsewhere.

Multiplicative Inverse of a Matrix If A and X are n x n matrices, and AX = XA = I, then X is the multiplicative inverse of A, written:

Show that matrices A and B are multiplicative inverses. A = B = 3 – – = (3)(0.1) + (–1)(–0.7) (3)(0.1) + (–1)(0.3) (7)(0.1) + (1)(–0.7) (7)(0.1) + (1)(0.3) AB = I, so B is the multiplicative inverse of A.

Check Understanding P. 196 # 1B

Determinate of a 2 x 2 matrix The Determinate of a 2 x 2 matrix is ad – bc.

Evaluate each determinant. a. det b. det c. det 7 8 –5 –9 4 –3 5 6 a –b b a = = (7)(–9) – (8)(–5) = – –5 –9 = = (4)(6) – (–3)(5) = 39 4 –3 5 6 = = (a)(a) – (–b)(b) = a 2 + b 2 a –b b a

Check Understanding P B and C

Inverse of a 2x2 Matrix Let A =. If det A  0, then has an inverse. If det A  0, then

Determine whether each matrix has an inverse. If it does, find it. Find det X. ad – bc = (12)(3) – (4)(9) Simplify. = Since det X = 0, the inverse of X does not exist. Find det Y. ad – bc = (6)(20) – (5)(25) Simplify. = – Since the determinant 0, the inverse of Y exists.=/ a. X = b. Y =

(continued) = – 20 –5 Substitute –5 for the –25 6 determinant = Multiply. –4 1 5 –1.2 Y –1 = 20 –5 Change signs. –25 6 Switch positions. 20 –5 Use the determinant to –25 6 write the inverse. = 1 det Y 1 det Y

Check Understanding P. 197 # 3A

Solve X = for the matrix X. The matrix equation has the form AX = B. First find A – –7 A –1 = 1 ad – bc d –b –c a Use the definition of inverse. = 1 (9)(11) – (25)(4) 11 –25 –4 9 Substitute. = – –9 Simplify. Use the equation X = A –1 B. X = – –9 Substitute. 3 –7

(continued) = = (–11)(3) + (25)(–7) (4)(3) + (–9)(–7) Multiply and simplify. – Check: X = Use the original equation. 3 – Substitute. 3 –7 – Multiply and simplify. 3 –7 9(–208) + 25(75) 4(–208) + 11(75) = 3 –7 3 –7

Homework P. 199 # 2-24 even