# Matrix Solution of Equations

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1 Contents 8 Matrix Solution of Equations 8.1 Solution by Cramer s Rule Solution by Inverse Matrix Method Solution by Gauss Elimination 22 Learning outcomes In this Workbook you will learn to apply your knowledge of matrices to solve systems of linear equations. Such systems of equations arise very often in mathematics, science and engineering. Three basic techniques are outlined, Cramer's method, the inverse matrix approach and the Gauss elimination method. The Gauss elimination method is, by far, the most widely used (since it can be applied to all systems of linear equations). However, you will learn that, for certain (usually small) systems of linear equations the other two techniques may be better.

2 Solution by Cramer s Rule 8.1 Introduction The need to solve systems of linear equations arises frequently in engineering. The analysis of electric circuits and the control of systems are two examples. Cramer s rule for solving such systems involves the calculation of determinants and their ratio. For systems containing only a few equations it is a useful method of solution. Prerequisites Before starting this Section you should... Learning Outcomes On completion you should be able to... be able to evaluate 2 2 and 3 3 determinants state and apply Cramer s rule to find the solution of two simultaneous linear equations state and apply Cramer s rule to find the solution of three simultaneous linear equations recognise cases where the solution is not unique or a solution does not exist 2 HELM (2008): Workbook 8: Matrix Solution of Equations

3 1. Solving two equations in two unknowns If we have one linear equation ax = b in which the unknown is x and a and b are constants then there are just three possibilities: a 0 then x = b a = a 1 b. In this case the equation ax = b has a unique solution for x. a = 0, b = 0 then the equation ax = b becomes 0 = 0 and any value of x will do. In this case the equation ax = b has infinitely many solutions. a = 0 and b 0 then ax = b becomes 0 = b which is a contradiction. In this case the equation ax = b has no solution for x. What happens if we have more than one equation and more than one unknown? We shall find that the solutions to such systems can be characterised in a manner similar to that occurring for a single equation; that is, a system may have a unique solution, an infinity of solutions or no solution at all. In this Section we examine a method, known as Cramer s rule and employing determinants, for solving systems of linear equations. Consider the equations ax + by = e cx + dy = f (1) (2) where a, b, c, d, e, f are given numbers. The variables x and y are unknowns we wish to find. The pairs of values of x and y which simultaneously satisfy both equations are called solutions. Simple algebra will eliminate the variable y between these equations. We multiply equation (1) by d, equation (2) by b and subtract: first, (1) d adx + bdy = ed then, (2) b bcx + bdy = bf (we multiplied in this way to make the coefficients of y equal.) Now subtract to obtain (ad bc)x = ed bf (3) Task Starting with equations (1) and (2) above, eliminate x. HELM (2008): Section 8.1: Solution by Cramer s Rule 3

4 Answer Multiply equation (1) by c and equation (2) by a to obtain acx + bcy = ec and acx + ady = af. Now subtract to obtain (bc ad)y = ec af If we multiply this last equation in the Task above by 1 we obtain (ad bc)y = af ec (4) Dividing equations (3) and (4) by ad bc we obtain the solutions x = ed bf ad bc, af ec y = ad bc (5) There is of course one proviso: if ad bc = 0 then neither x nor y has a defined value. If we choose to express these solutions in terms of determinants we have the formulation for the solution of simultaneous equations known as Cramer s rule. If we define as the determinant equations a c b d and provided 0 then the unique solution of the ax + by = e cx + dy = f is by (5) given by x = x, y = y where x = e f b d, y = a c e f Now is the determinant of coefficients ( ) on the left-hand sides of the equations. In the expression a x the coefficients of x (i.e. which is column 1 of ) are replaced by the terms on the c ( ) e right-hand sides of the equations (i.e. by ). Similarly in f y the coefficients of y (column 2 of ) are replaced by the terms on the right-hand sides of the equations. 4 HELM (2008): Workbook 8: Matrix Solution of Equations

5 The unique solution to the equations: is given by: in which = a c Key Point 1 Cramer s Rule for Two Equations ax + by = e cx + dy = f x = x, b d x = e f If = 0 this method of solution cannot be used. y = y b d, y = a c e f Task Use Cramer s rule to solve the simultaneous equations 2x + y = 7 3x 4y = 5 Answer Calculating = = 11. Since 0 we can proceed with Cramer s solution. = = 11 x = , y = ( 28 5) (10 21) i.e. x =, y = implying: x = 33 ( 11) ( 11) 11 = 3, y = = 1. You can check by direct substitution that these are the exact solutions to the equations. HELM (2008): Section 8.1: Solution by Cramer s Rule 5

6 Task Use Cramer s rule to solve the equations (a) 2x 3y = 6 4x 6y = 12 (b) 2x 3y = 6 4x 6y = 10 Answer You should have checked first, since = 12 ( 12) = 0. Hence there is no unique solution in either case. In the system (a) the second equation is twice the first so there are infinitely many solutions. (Here we can give y any value we wish, t say; but then the x value is always (6 + 3t)/2. So for each value of t there are values for x and y which simultaneously satisfy both equations. There is an infinite number of possible solutions). In (b) the equations are inconsistent (since the first is 2x 3y = 6 and the second is 2x 3y = 5 which is not possible). Hence there are no solutions. Notation For ease of generalisation to larger systems we write the two-equation system in a different notation: a 11 x 1 + a 12 x 2 = b 1 a 21 x 1 + a 22 x 2 = b 2 Here the unknowns are x 1 and x 2, the right-hand sides are b 1 and b 2 and the coefficients are a ij where, for example, a 21 is the coefficient of x 1 in equation two. In general, a ij is the coefficient of x j in equation i. Cramer s rule can then be stated as follows: If a 11 a 12 a 21 a 22 0, then the equations a 11 x 1 + a 12 x 2 = b 1 a 21 x 1 + a 22 x 2 = b 2 have solution b 1 a 12 b 2 a 22 x 1 = a 11 a 12 a 21 a 22, x 2 = a 11 b 1 a 21 b 2 a 11 a 12. a 21 a 22 6 HELM (2008): Workbook 8: Matrix Solution of Equations

7 2. Solving three equations in three unknowns Cramer s rule can be extended to larger systems of simultaneous equations but the calculational effort increases rapidly as the size of the system increases. We quote Cramer s rule for a system of three equations. Key Point 2 Cramer s Rule for Three Equations The unique solution to the system of equations: a 11 x 1 + a 12 x 2 + a 13 x 3 = b 1 a 21 x 1 + a 22 x 2 + a 23 x 3 = b 2 a 31 x 1 + a 32 x 2 + a 33 x 3 = b 3 is in which and x1 = b 1 a 12 a 13 b 2 a 22 a 23 b 3 a 32 a 33 x 1 = x 1, x 2 = x 2, x 3 = x 3 = x2 = a 11 a 12 a 13 a 21 a 22 a 23 a 31 a 32 a 33 a 11 b 1 a 13 a 21 b 2 a 23 a 31 b 3 a 33 x3 = a 11 a 12 b 1 a 21 a 22 b 2 a 31 a 32 b 3 If = 0 this method of solution cannot be used. Notice that the structure of the fractions is similar to that for the two-equation case. For example, the determinant forming the numerator of x 1 is obtained from the determinant of coefficients,, by replacing the first column by the right-hand sides of the equations. Notice too the increase in calculation: in the two-equation case we had to evaluate three 2 2 determinants, whereas in the three-equation case we have to evaluate four 3 3 determinants. Hence Cramer s rule is not really practicable for larger systems. HELM (2008): Section 8.1: Solution by Cramer s Rule 7

8 Task Use Cramer s rule to solve the system x 1 2x 2 + x 3 = 3 2x 1 + x 2 x 3 = 5 3x 1 x 2 + 2x 3 = 12. First check that 0: Answer = Expanding along the top row, = ( 2) = 1 (2 1) + 2 (4 + 3) + 1 ( 2 3) = = 10 Now find the value of x 1. First write down the expression for x 1 in terms of determinants: Answer x 1 = Now calculate x 1 explicitly: 8 HELM (2008): Workbook 8: Matrix Solution of Equations

9 Answer The numerator is found by expanding along the top row to be ( 2) = ( 17) = 30 Hence x 1 = = 3 In a similar way find the values of x 2 and x 3 : Answer x 2 = 1 10 = { } = 1 { } = 1 10 x 3 = 1 { ( 2) = 1 { ( 5)} = } HELM (2008): Section 8.1: Solution by Cramer s Rule 9

10 Engineering Example 1 Stresses and strains on a section of material Introduction An important engineering problem is to determine the effect on materials of different types of loading. One way of measuring the effects is through the strain or fractional change in dimensions in the material which can be measured using a strain gauge. Problem in words In a homogeneous, isotropic and linearly elastic material, the strains (i.e. fractional displacements) on a section of the material, represented by ε x, ε y, ε z for the x-, y-, z-directions respectively, can be related to the stresses (i.e. force per unit area), σ x, σ y, σ z by the following system of equations. ε x = 1 E ( σ x vσ y vσ z ) ε y = 1 E ( vσ x +σ y vσ z ) ε z = 1 E ( vσ x vσ y +σ z ) where E is the modulus of elasticity (also called Young s modulus) and v is Poisson s ratio which relates the lateral strain to the axial strain. Find expressions for the stresses σ x, σ y, σ z, in terms of the strains ε x, ε y, and ε z. Mathematical statement of problem The given system of equations can be written as a matrix equation: ε x ε y = 1 1 v v σ x v 1 v σ y E ε z v v 1 σ z We can write this equation as ε = 1 E Aσ ε x where ε = ε y, A = ε z 1 v v v 1 v v v 1 and σ = This matrix equation must be solved to find the vector σ in terms of the vector ε and the inverse of the matrix A. σ x σ y σ z 10 HELM (2008): Workbook 8: Matrix Solution of Equations

11 Mathematical analysis ε = 1 E Aσ Multiplying both sides of the expression by E we get Eε = Aσ Multiplying both sides by A 1 we find that: A 1 Eε = A 1 Aσ But A 1 A = I so this becomes σ = EA 1 ε To find expressions for the stresses σ x, σ y, σ z, in terms of the strains ε x, ε y and ε z, we must find the inverse of the matrix A. 1 v v To find the inverse of v 1 v we first find the matrix of minors which is: v v 1 1 v v 1 v v v 1 v v 1 v v v v 1 1 v v 1 1 v v v We then apply the pattern of signs: to obtain the matrix of cofactors 1 v 2 v + v 2 v 2 + v v + v 2 1 v 2 v + v 2. v 2 + v v + v 2 1 v 2 v 1 v v 1 v v v = 1 v v 1 1 v 2 v v 2 v 2 + v v v 2 1 v 2 v v 2 v 2 + v v v 2 1 v 2 To find the adjoint we take the transpose of the above, (which is the same as the original matrix since the matrix is symmetric) 1 v 2 v + v 2 v 2 + v v + v 2 1 v 2 v + v 2. v 2 + v v + v 2 1 v 2 The determinant of the original matrix is 1 (1 v 2 ) v(v + v 2 ) v(v 2 + v) = 1 3v 2 2v 3.. HELM (2008): Section 8.1: Solution by Cramer s Rule 11

12 Finally we divide the adjoint by the determinant to find the inverse, giving 1 v 1 2 v + v 2 v + v 2 v + v 2 1 v 2 v + v v 2 2v 3 v + v 2 v + v 2 1 v 2 σ x 1 v Now we found that σ = EA 1 ε so σ y E 2 v + v 2 v + v 2 ε x = v + v 2 1 v 2 v + v 2 ε 1 3v σ 2 2v 3 y z v + v 2 v + v 2 1 v 2 ε z We can write this matrix equation as 3 equations relating the stresses σ x, σ y, σ z, in terms of the strains ε x, ε y and ε z, by multiplying out this matrix expression, giving: σ x = σ y = σ z = E ( ) (1 v 2 )ε 1 3v 2 2v 3 x + (v + v 2 )ε y + (v + v 2 )ε z E ( ) (v + v 2 )ε 1 3v 2 2v 3 x + (1 v 2 )ε y + (v + v 2 )ε z E ( ) (v + v 2 )ε 1 3v 2 2v 3 x + (v + v 2 )ε y + (1 v 2 )ε z Interpretation Matrix manipulation has been used to transform three simultaneous equations relating strain to stress into simultaneous equations relating stress to strain in terms of the elastic constants. These would be useful for deducing the applied stress if the strains are known. The original equations enable calculation of strains if the applied stresses are known. 1. Solve the following using Cramer s rule: (a) 2x 3y = 1 4x + 4y = 2 (b) Exercises 2x 5y = 2 4x + 10y = 1 2. Using Cramer s rule obtain the solutions to the following sets of equations: (c) 6x y = 0 2x 4y = 1 (a) 2x 1 + x 2 x 3 = 0 x 1 + x 3 = 4 x 1 + x 2 + x 3 = 0 (b) x 1 x 2 + x 3 = 1 x 1 + x 3 = 1 x 1 + x 2 x 3 = 0 Answers 1. (a) x = 1 2, y = 0 (b) = 0, no solution (c) x = 1 22, y = (a) x 1 = 8 3, x 2 = 4, x 3 = 4 3 (b) x 1 = 1 2, x 2 = 1, x 3 = HELM (2008): Workbook 8: Matrix Solution of Equations

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