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Demonstration of the function "root"

by R. Grothmann

"root" is an equation solver for interactive use. It can solve an equation for one variable, and will assign the result to this variable.

We first take a simple example.

>longformat; a:=2; x:=1; root("x^2-a",x)

The solution for x is assigned to the global variable x.


We could also solve for the variable "a" starting with a different value of the variable "a".

>a:=3; root("x^2-a",a)

For a more complex example, we take the computation of interest rates for a loan (or a savings account).

Assume you get K (K>0) at time 0 and pay P (P<0) at each period, starting from period 1 to period n-1. You then have a final depth F (F<0). What is the interest rate?

>ex &= K*f^n+(f^(n-i0+1)-1)/(f-1)*f^i1*P+F
                   i1   n - i0 + 1
                  f   (f           - 1) P    n
                  ----------------------- + f  K + F
                           f - 1

After we set up the expression, we initialize the variables with values. We take an approximation for f (8 %). This time we have to pay 1000 each month. We are done (F=0) after 120 month.

We set up starting values for all variables in the formula.

>K:=100000; n:=120; f:=1.08^(1/12); P:=-1000; i0:=1; i1:=0; F:=0;

Then we solve for the correct interest rate "f".


To compute the effective interest rate per year, we must take f^12 and compute the interest rate in %.


Assume we stop paying after 119 months, and the interest rate is 8%. How much would be left after 120 month?

>f:=1.08^(1/12); i1:=1; "Dept left:"|print(-root(ex,F),2)
Dept left:  34609.30

How long would it take to pay the loan at 8%?

>F:=0; i1:=0; "Payed after "|print(root(ex,n)/12,1,0)|" years"
Payed after 13.4 years