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);}