exp

Calculates the natural exponential of a number

```#include <math.h>
double exp ( double x  );
float expf ( float x  );
long double expl ( long double x  );
```

The return value of the exp( ) function is e raised to the power of the function's argument, or ex, where e is Euler's number, 2.718281.... If the result is beyond the range of the function's type, a range error occurs. The natural exponential function exp( ) is the inverse of the natural logarithm function, log( ).

#### Example

```/* Amount owed = principal * e**(interest_rate * time) */
int principal = 10000;      // Initial debt is ten thousand dollars.
int balance = 0;
double rate = 0.055;        // Interest rate is 5.5% annually.
double time = 1.5;          // Period is eighteen months.

balance = principal * exp( rate * time );
printf("Invest %d dollars at %.1f%% compound interest, and "
"in %.1f years you'll have %d dollars.\n",
principal, rate*100.0, time, balance );
```

This code produces the following output:

```Invest 10000 dollars at 5.5% compound interest, and in 1.5 years
you'll have 10859 dollars.
``` 