When I am using the formula for calculating the fibonacci number, I am getting WA but with Matrix exponentiation I got right answer.

Can anyone explain why?

#include<bits/stdc++.h>

using namespace std;

#define ll long long

#define mod 1000000007

ll fib(ll n) {

double phi = (1 + sqrt(5)) / 2;

return round(pow(phi, n) / sqrt(5));

}

int main()

{

ll t;

cin>>t;

```
while(t--)
{
ll n,k;
cin>>n>>k;
ll x=n/k, y=n%k, ans=0;
x%=mod;
if(x>0)
{
ans+=(x*(fib(k+1)-1))%mod;
ans%=mod;
}
if(y>0)
{
ans+=((fib(y+1)-1))%mod;
ans%=mod;
}
cout<<ans<<endl;
}
```

}