Recursion

Factorial

#include<stdio.h>
unsigned long fact(int );

void main()
{
  int a;
  printf("\nenter value of n ");
  scanf("%d",&a);
  printf("\n%d",fact(a));
  /*b=sum(a);
  printf("\n%d",b);*/
}

unsigned long fact(int a)
{
  if(a==1)
  return 1;
  return(a*fact(a-1));
}

Fibonacci 

#include<stdio.h>

void main()
{
  int a=0,i=0;//b=1,n,i=2,sum=0;
  printf("\nenter length");
  scanf("%d",&a);
  for(i=1;i<=a;i++)
  printf("\n%d",fib(i));
  }
 
int fib(int a)
{
  if(a==1)
  return 0;
  else if(a==2)
  return 1;
  return (fib(a-1)+fib(a-2));
}

GCD

//greatest common devider
#include<stdio.h>

void main()
{
  int a,b;
  printf("\nenter values ");
  scanf("%d%d",&a,&b);
  printf("%d",gcd(a,b));
 
}

int gcd(int a,int b)
{
  if(a==b)
  return a;
else if(a%b==0)
  return b;
  else if(b%a==0)
  return a;
  else if(a>b)
  return gcd(a%b,b);
  else
  return gcd(a,b%a);
}