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Further Pure 1 Inverse Matrices

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Reminder from lesson 1 Note that any matrix multiplied by the identity matrix is itself. And any matrix multiplied by the zero matrix is the zero matrix.

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Inverse Matrix All operations have an opposite. We discussed in lesson 2 about using matrices to perform transformations. An inverse matrix will undo the transformation and return you to where you started. If a matrix is called A, then its inverse is known as A -1. In lesson 1 we briefly met the concept of an identity matrix (seen on first slide). So if multiplying A by A -1 returns you to where you started and multiplying by the identity matrix leaves you where you are, we can conclude that AA -1 = A -1 A = I

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Challenge 1 You need to know the general formula for the inverse of a 2 × 2 matrix. Can you find the inverse to the following matrix? Use the property TT -1 = I From this we can form 2 pairs of simultaneous equations. 2p + 3q = 1 2r + 3s= 0 p + 4q = 0 r + 4s= 1

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Challenge 1 The solutions to these equations are So the inverse of T is

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Challenge 1 Lets now take any 2 × 2 matrix. Can you use T and T -1 to find the inverse of M and hence the general formula for the inverse of any 2 × 2 matrix?

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The Determinant of a 2 × 2 matrix We have just found the general equation for the inverse of any 2 × 2 matrix. The Δ symbol is a capital delta and will always be a numerical value. The value can be calculated from the matrix and is known as the determinant of the matrix. Using T and T -1 can you spot how to calculate it?

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The Determinant of a 2 × 2 matrix To calculate the determinant of a matrix M you multiply a by d and subtract b by c. Below is the official notation. If the det is zero then the inverse does not exist and the matrix is known as singular. If the det is not zero then the inverse does exist and the matrix is known as non-singular. Note: Only square matrices have inverses.

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Challenge 2 This is above the scope of the course and not required for you to do. However it is a challenging question that will test your algebraic manipulation skills. Can you find the inverse of M using the identity below and the method we used a few slides ago. This will also prove where the formula for the determinant comes from.

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Questions Find the inverse of the following matrices.

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Inverse of a product Find the inverse of AB. Lets call the inverse of AB, X. So as we already know X(AB) = I First multiply by B -1 X(AB)B -1 = I×B -1 XA = B -1 Next multiply by A -1 XAA -1 = B -1 A -1 X = B -1 A -1 This is an important result that you need to know (AB) -1 = B -1 A -1

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Properties of the determinant The orange square is an enlargement of the black square by a scale factor 2. What is the area of the object? Area = 9 units 2 What is the area of the image? Area = 36 units 2 The transformation performed can be described by the following matrix What do you notice about the determinant of the matrix and the enlargement shown. The determinant of a matrix indicates the scale factor of the area of enlargement. The det T is known as the signed scale factor as it can be negative. The negative signifies that the rotation direction has been reversed.

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Task Can you explain how we know that the area of any shape rotated θ degrees anti- clockwise about the origin remains the same.

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Matrices with det = 0 The determinant of a matrix tells us the scale factor of the areas` enlargement. What would be the area of a shape transformed by a matrix with det = 0? The area would be 0. All the points will have been transformed so what will the image look like? The image will be a straight line. We can see an example of this on the next slide.

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Example 1 Lets start with a rectangle on a 2D pair of axes. We can write the co-ordinates of the vertices in matrix form. Next transform the object using a matrix with a det = 0 The image becomes a series of points that are in a straight line.

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Example 1 In fact although we used a rectangle for the example any point in the plane will transform to the line. From the diagram its clear to see what the equation of the line will be. y = x

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Example 1 We can reach the same result as the last slide using an algebraic method. Lets look at the general co- ordinate (x,y). Under the transformation we get the co-ordinates (x`,y`) Using matrix multiplication we can see that. x + 2y = x` x + 2y = y` From this we get y` = x + 2y = x` Or y = x

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Example 2 The plane is transformed by the matrix. Show that the whole plane is mapped to a straight line and find the equation of this line. Using matrix multiplication gives us the simultaneous equations. x` = 2x – y y` = -4x + 2y From the equations we get y` = -2(2x – y) = -2x` All the points will map to the line y = -2x

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Example 2 All points in a plane transform to a straight line. This is because there are infinitely many lines that transform to a single point.

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Example 3 For the matrix T find the equation of the line of points that map to (5,-10). We use matrix multiplication to find what equations will be equal to the co-ordinate (5,-10) This gives us the equations 2x – y = 5 -4x + y = -10 These two equations give the exact same information. 2x – y = 5

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Summary 1 The inverse of a matrix Is Where

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Summary 2 MM -1 = M -1 M = I X = B -1 A -1

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