10.6.2 Solving Systems of Equations

You can also use the commands described above to solve systems of simultaneous equations. Just create a vector of equations, then specify a vector of variables for which to solve. (You can omit the surrounding brackets when entering the vector of variables at the prompt.)

For example, putting ‘[x + y = a, x - y = b]’ on the stack and typing a S x,y RET produces the vector of solutions ‘[x = a - (a-b)/2, y = (a-b)/2]’. The result vector will have the same length as the variables vector, and the variables will be listed in the same order there. Note that the solutions are not always simplified as far as possible; the solution for ‘x’ here could be improved by an application of the a n command.

Calc’s algorithm works by trying to eliminate one variable at a time by solving one of the equations for that variable and then substituting into the other equations. Calc will try all the possibilities, but you can speed things up by noting that Calc first tries to eliminate the first variable with the first equation, then the second variable with the second equation, and so on. It also helps to put the simpler (e.g., more linear) equations toward the front of the list. Calc’s algorithm will solve any system of linear equations, and also many kinds of nonlinear systems.

Normally there will be as many variables as equations. If you give fewer variables than equations (an “over-determined” system of equations), Calc will find a partial solution. For example, typing a S y RET with the above system of equations would produce ‘[y = a - x]’. There are now several ways to express this solution in terms of the original variables; Calc uses the first one that it finds. You can control the choice by adding variable specifiers of the form ‘elim(v)’ to the variables list. This says that v should be eliminated from the equations; the variable will not appear at all in the solution. For example, typing a S y,elim(x) would yield ‘[y = a - (b+a)/2]’.

If the variables list contains only elim specifiers, Calc simply eliminates those variables from the equations and then returns the resulting set of equations. For example, a S elim(x) produces ‘[a - 2 y = b]’. Every variable eliminated will reduce the number of equations in the system by one.

Again, a S gives you one solution to the system of equations. If there are several solutions, you can use H a S to get a general family of solutions, or, if there is a finite number of solutions, you can use a P to get a list. (In the latter case, the result will take the form of a matrix where the rows are different solutions and the columns correspond to the variables you requested.)

Another way to deal with certain kinds of overdetermined systems of equations is the a F command, which does least-squares fitting to satisfy the equations. See Curve Fitting.