Function Calls
 Functions are small bits of code that can be called anywhere in the code.
 Function usually solve small problems.
 They can take input
 They can produce output
 The math library provides a set of functions
 Let's look at the square root function.
 reference
 First, we see that it has changed over the years.
 C++11 and beyond it needs
#include < cmath>
 It takes a single parameter or input value.
 It returns a single value.
 Note a function prototype

double sqrt (double x);
 Lists the return type
 The function name.
 At least the types of any parameters or arguments.
 The standard library supplies a large number of functions.
 If you need a function, searching is probably the best way to find it.
 Beware you might need to add a library for some.
 a list
 Memorizing all of the math functions is not a good idea.
 Our author discusses several functions

int abs(int)

float fabs(float)

double cos(double)
 (but float, long double, ...
 the angle is expected to be in radians
 But
M_PI
is a constant from cmath

float pow(float base, float exponent)
 Eventually we will write our own functions.
 /usr/include/math.h has additional constants.
 We will need
M_E
for the next problem.
 An approximation for n! is $e^{n}n^n\sqrt{2\pi n}$.
 Compute 15! directly and using this approximation.
 This is problem 3 from page 134