Built-in functions
Any programming language includes many built-in functions that can be used in arithmetic expressions. To use additional functions, you often have to include maths library: 
#include<cmath> 
For example, the most commonly used standard mathematical functions and their writing in C and C ++
 abs(i) - |i|  - absolute value of an integer i 
 fabs(x) - |x|  - absolute value of a real числа x
 sqrt(x) -  \(\sqrt x\)
 pow(x,y) - xy
 
Remember!
The function argument is always written in brackets.
For these functions to work, you need to include maths library:  #include<cmath>