Đa thức, đạo hàm C

#include <stdio.h>
#include <math.h>
#define ll long long
int main()
{
    ll n;
    printf("Nhap so bac cua pt: ");
    scanf("%lld",&n);

//-----
//Nhap pt
    ll a[n+1];
    for (ll i=n; i>=0; i--){
        printf("Nhap he so cua x^%lld: ",i);
        scanf("%lld",&a[i]);
    }

//-----
//In pt
    printf("pt P(x): ");
    for (ll i=n; i>0; i--){
        printf("%lldx^%lld + ",a[i],i);
    }
    printf("%lld\n",a[0]);

//-----
//Tinh dao ham
    ll dh_a[n+1];
    dh_a[n]=0;
    for (ll i=0; i<n; i++){
        dh_a[i]=a[i+1]*(i+1);
    }
    printf("dh P(x): ");
    for (ll i=n-1; i>0; i--){
        printf("%lldx^%lld + ",dh_a[i],i);
    }
    printf("%lld\n",dh_a[0]);

//-----
//solve
    double x;
    printf("Nhap x: ");
    scanf("%lf",&x);
    double s=2021;
    for (ll i=0; i<n; i++){
        s+=a[i]*pow(x,i);
        s+=dh_a[i]*pow(x,i);
    }
    s=sqrt(s);
    printf("S= %lf",s);



    return 0;
}