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### 2.3 Rational Expression

Command: num expr

The function `num` takes a rational expression as input and returns a numerator of the expression.

```e25 : num((x^2+y^2)/(x^2-y^2));

2    2
e25: - x  - y

e26 : num(7/4);

e26: 7

e27 : num(7/(4/3));

e27: 21
```
Operator: denom rational-expression

The Jacal command `denom` is used to obtain the denominator of a rational expression.

```e26 : denom(4/5);

e26: 5
```
Command: listofvars expr

The command `listofvars` takes as input a rational expression and returns a list of the variables that occur in that expression.

```e7 : listofvars(x^2+y^3);

e7: [x, y]

e8 : listofvars((x^2+y^3)/(2*x^7+y*x+z));

e8: [z, x, y]
```
Command: imagpart z

Returns the coefficient of `%i` in expression z;

Command: realpart z

Returns all but the coefficient of `%i` in expression z;

Command: abs z
Command: cabs z

`| z |`

Returns the square root of the sum of the squares of the `realpart` and the `imagpart` of z.