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PRECALCULUS 2 Determinants, Inverse Matrices & Solving

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**December DO NOW: TAKE OUT PAPER OR A NOTEBOOK!!!!**

CW: INVERSE POWER POINT!!! HW: INVERSE wksts. SWBAT: Identify the inverse of a 2x2 matrix Identify the inverse of a 3 x 3 matrix

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**- (-5 * 2) (3 * 4) 12 - (-10) 22 Notice the different symbol:**

Finding Determinants of Matrices Notice the different symbol: the straight lines tell you to find the determinant!! - = (3 * 4) (-5 * 2) (-10) = = 22

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**= [(2)(-2)(2) + (0)(5)(-1) + (3)(1)(4)]**

Finding Determinants of Matrices 2 1 -1 -2 4 = [(2)(-2)(2) + (0)(5)(-1) + (3)(1)(4)] - [(3)(-2)(-1) + (2)(5)(4) + (0)(1)(2)] = [ ] - [ ] = 4 – = -42

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**Using matrix equations**

Inverse Matrix: 2 x 2 In words: Take the original matrix. Switch a and d. Change the signs of b and c. Multiply the new matrix by 1 over the determinant of the original matrix.

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**Using matrix equations**

Example: Find the inverse of A. =

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**Find the inverse matrix.**

Reloaded Matrix A Det A = 8(2) – (-5)(-3) = 16 – 15 = 1 = =

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**Using matrix equations**

Identity matrix: Square matrix with 1’s on the diagonal and zeros everywhere else 2 x 2 identity matrix 3 x 3 identity matrix The identity matrix is to matrix multiplication as ___ is to regular multiplication!!!! 1

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**= = Mathematically, IA = A and AI = A !! Multiply:**

So, the identity matrix multiplied by any matrix lets the “any” matrix keep its identity! Mathematically, IA = A and AI = A !!

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**What happens when you multiply a matrix by its inverse?**

1st: What happens when you multiply a number by its inverse? A & B are inverses. Multiply them. = So, AA-1 = I

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**X X X X Why do we need to know all this? To Solve Problems!**

Solve for Matrix X. = X We need to “undo” the coefficient matrix. Multiply it by its INVERSE! = X X = X =

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**Using matrix equations**

You can take a system of equations and write it with matrices!!! 3x + 2y = 11 2x + y = 8 = becomes Answer matrix Coefficient matrix Variable matrix

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**Using matrix equations**

= Example: Solve for x and y . Let A be the coefficient matrix. Multiply both sides of the equation by the inverse of A. -1 = = = = =

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**Using matrix equations**

It works!!!! Wow!!!! x = 5; y = -2 3x + 2y = 11 2x + y = 8 3(5) + 2(-2) = 11 2(5) + (-2) = 8 Check:

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You Try… Solve: 4x + 6y = 14 2x – 5y = -9 (1/2, 2)

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