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1.6 In this section we shall establish more results about systems of linear equations
and invertibility of matrices. Our work will lead to a new method for solving n
FURTHER RESULTS ON equations in n unknowns.
SYSTEMS OF
EQUATIONS AND
INVERTIBILITY
A Basic Theorem
In Section 1.1 we made the statement (based on Figure 1.1.1) that every linear system has no solutions, or has one solution, or has
infinitely many solutions. We are now in a position to prove this fundamental result.
THEOREM 1.6.1
Every system of linear equations has no solutions, or has exactly one solution, or has infinitely many solutions.
Proof If is a system of linear equations, exactly one of the following is true: (a) the system has no solutions, (b) the system
has exactly one solution, or (c) the system has more than one solution. The proof will be complete if we can show that the system
has infinitely many solutions in case (c).
Assume that has more than one solution, and let , where and are any two distinct solutions. Because
and are distinct, the matrix is nonzero; moreover,
If we now let k be any scalar, then
But this says that is a solution of . Since is nonzero and there are infinitely many choices for k, the system
has infinitely many solutions.
Solving Linear Systems by Matrix Inversion
Thus far, we have studied two methods for solving linear systems: Gaussian elimination and Gauss–Jordan elimination. The
following theorem provides a new method for solving certain linear systems.
THEOREM 1.6.2
If A is an invertible matrix, then for each matrix b, the system of equations has exactly one solution, namely,
.
