E-Book Content
ELEMENTARY LINEAR ALGEBRA
K. R. MATTHEWS DEPARTMENT OF MATHEMATICS
UNIVERSITY OF QUEENSLAND
First Printing, 1991
Chapter 1
LINEAR EQUATIONS 1.1
Introduction to linear equations
A linear equation in n unknowns x1 , x2 , · · · , xn is an equation of the form a1 x1 + a2 x2 + · · · + an xn = b, where a1 , a2 , . . . , an , b are given real numbers. For example, with x and y instead of x1 and x2 , the linear equation 2x + 3y = 6 describes the line passing through the points (3, 0) and (0, 2). Similarly, with x, y and z instead of x1 , x2 and x3 , the linear equation 2x + 3y + 4z = 12 describes the plane passing through the points (6, 0, 0), (0, 4, 0), (0, 0, 3). A system of m linear equations in n unknowns x1 , x2 , · · · , xn is a family of linear equations
a11 x1 + a12 x2 + · · · + a1n xn = b1
a21 x1 + a22 x2 + · · · + a2n xn = b2 .. .
am1 x1 + am2 x2 + · · · + amn xn = bm . We wish to determine if such a system has a solution, that is to find out if there exist numbers x1 , x2 , · · · , xn which satisfy each of the equations simultaneously. We say that the system is consistent if it has a solution. Otherwise the system is called inconsistent. 1
2
CHAPTER 1. LINEAR EQUATIONS Note that the above system can be written concisely as n X
aij xj = bi ,
j=1
The matrix
a11 a21 .. .
a12 a22
i = 1, 2, · · · , m.
··· ···
a1n a2n .. .
am1 am2 · · · amn
is called the coefficient matrix of the system, while the matrix a11 a12 · · · a1n b1 a21 a22 · · · a2n b2 .. .. .. . . . am1 am2 · · · amn bm
is called the augmented matrix of the system. Geometrically, solving a system of linear equations in two (or three) unknowns is equivalent to determining whether