Given an array in range 1..n a value is repeated twice and a value is missing. Find that duplicate in O(n)

An array is given with values in range from 1...n and a number is repeated twice and a number is missing in that range(1..n).
Following program finds the missing value and repeated element in O(n).

/**
 * @author vikky.agrawal
 */


public class FindDuplicate {

    /**
     * @param args
     */
    public static void main(String[] args) {
        FindDuplicate obj = new FindDuplicate();
        int arr[]= new int[]{1,2,3,4,5,6,6,8,9};
        
        obj.findDuplicate(arr, 9);

    }
    
    public void findDuplicate(int[] arr, int n){
        
        if(n<=0){
            return;
        }
        
        int sum=(n*(n+1))/2;
        int squareSum = (n*(n+1)*((2*n)+1))/6;
        
        int sumofArray=0;
        int squareSumofArray=0;
        
        for(int a: arr){
            sumofArray+=a;
            squareSumofArray+=(a*a);
        }
        
        int a_b= sumofArray-sum; //a-b
        int a2_b2= squareSumofArray-squareSum; //a^2-b^2
        
        // (a+b)= (a^2-b^2)/(a-b)
        
        
        int aplusb=a2_b2/a_b;
        int a2=a_b+aplusb;
        int b= aplusb-(a2/2);
        
        System.out.println("Missing element is :"+b);
        System.out.println("Duplicate element is :"+(a2/2));
    }
    
 }