POWER SOLUTION

public int power(int base, int exponent) {
    if (exponent == 0) {
      return 1;
    }
    return base * power(base,exponent-1);
  }

FACTORIAL SOLUTION

public int factorial(int num) {
    if (num<=1) {
      return 1;
    }
    return num * factorial(num-1);
  }

PRODUCT OF ARRAY SOLUTION

public int productofArray(int A[], int N) 
    { 
        if (N <= 0) 
            return 1; 
        return (productofArray(A, N - 1) * A[N - 1]); 
    } 

RECURSIVE RANGE SOLUTION

public int recursiveRange(int num) {
     if (num<=0) {
       return 0;
     }
     return num + recursiveRange(num - 1);
   } 

FIBONACCI SOLUTION

 public int fib(int n) {
    if (n<0) {
      return -1;
    }
    if (n==0 || n==1) {
      return n;
    }
    return fib(n-1) + fib(n-2);
  }