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;
}