序列的第 k 个数

题目大意：

给你序列的前三项，这个序列要么是等差序列，要么是等比序列，求出第 \(k\) 项的值。

代码：

const int mod = 200907;

inline ll Read()
{
  ll x = 0, f = 1;
  char c = getchar();
  while (c != '-' && (c < '0' || c > '9')) c = getchar();
  if (c == '-') f = -f, c = getchar();
  while (c >= '0' && c <= '9') x = (x << 3) + (x << 1) + c - '0', c = getchar();
  return x * f;
}

int t;
ll a, b, c, n;

ll qpow(ll a, ll b)
{
  ll ans = 1;
  for (; b; b >>= 1, a = a * a % mod)
    if (b & 1) ans = ans * a % mod;
  return ans;
}

int main()
{
//  freopen(".in", "r", stdin);
//  freopen(".out", "w", stdout);
  for (t = Read(); t--; )
  {
    a = Read(), b = Read(), c = Read(), n = Read();
    if (b - a == c - b)
      printf ("%lld\n", (a + (b - a) * (n - 1) % mod) % mod);
    else printf ("%lld\n", a * qpow(b / a, n - 1) % mod);
  }
  return 0;
}