等差数列求和

等差数列求和公式：（a1 + an)  * n / 2

 an = a1 + (n - 1) * d;

private static void calc(int n) {
    if(n <= 0){
        System.out.println("非法输入");
        return;
    }
    int d = 3;
    int a1 = 2;
    int an = a1 + (n - 1) * d;
    int sum = (a1 + an) * n / 2;
    System.out.println(sum);
}