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C-programs on functions.

- Write a function to calculate factorial of a number.(Takes Something, Returns Something).
- Write a function to calculate area of a circle.(Takes Something, Returns Something)
- Write a program to print System Date.
- Write a function to calculate sum of first N natural numbers. (all four ways: TNRN, TSRN, TNRS, TSRS).
- Write a function to calculate sum of squares of first N natural numbers.(all four ways: TNRN, TSRN, TNRS, TSRS).
- Write a function to print Pascal triangle.
- Write a program to express a given number as a sum of two prime numbers. Print all possible solutions.