Single equations in one unknown using EXCEL

There are at least two methods to solve algebraic equations in EXCEL. The first one uses the 'Goal Seek' command. The other one, using 'Solver', is described for applications to finite difference methods for boundary value problems (link). Open a spreadsheet and put the following statement in cell B1.

=A1*A1 - 2*A1 - 8

Then under 'Tools' choose 'Goal Seek'. A small screen comes up and you fill in the spaces to show the following

Set cell $B$1
To value 0.
By changing cell A1

Then click OK. The answer appears in the spreadsheet.

-2.000007 4.1137E-06

Thus the solution found is ­2, with an error of a fraction of a percent. The test of whether the calculation is correct is shown in cell B1, which takes the value 4.1 x 10^-6. This is not zero, but it is small enough for most purposes. (You have to judge this.) If you want to decrease the tolerance, to make the solution more accurate, under 'Tools' and 'Options' choose 'Calculation', then in 'Maximum Change' add a few zeros in the middle (changing it from 0.001 to 0.000001), choose OK, and repeat the 'Goal Seek'. This time the answer is

-2 -1.376e-8

To get the other root, put the value 3 in cell A1 and choose 'Goal Seek'.