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Library functions are pre-written pieces of code that can be used in any program by simply including or importing the library. They are designed to perform specific tasks, such as mathematical calculations, file handling, or input/output operations. These functions are stored in external files, separate from the main program code, which makes it easier to maintain, debug, and reuse the code.

A "black box" is a device or system that has a clear input and output, but the internal workings are hidden, complex, or unknown. In the context of library functions, the term "black box" refers to the fact that a programmer can use a function without having to understand the internal workings or implementation details of that function. The programmer only needs to know the inputs, outputs, and purpose of the function.

Library functions provide encapsulation by hiding the implementation details and complexity of the function. This allows programmers to focus on their own code and use the library function without needing to understand how the function works internally. Abstraction is achieved as the user of the library function only needs to know the inputs required and the expected output of the function, without needing to understand the complexity inside the function.

Library functions promote modularity by separating the function's code from the main program code. This allows different parts of a program to be developed independently, making it easier to maintain and debug the code. Additionally, reusability is a key feature of library functions, as the same function can be used in multiple programs without the need to rewrite or duplicate the code, reducing the chances of errors and saving time. In conclusion, library functions are like "black boxes" because they offer encapsulation, abstraction, modularity, and reusability. By hiding the internal workings and providing clear inputs and outputs, library functions allow programmers to easily use them in their code without needing to understand their implementation details.