#include<stdio.h>
#include<conio.h>
#include<math.h>

float deQuy(float x, int n)
{
	if(n == 1)
	{
		return x;
	}
	return deQuy(x, n - 1) + pow(x, n);
}

float deQuyDuoi(float x, int n, float y = 0)
{
	if(n == 0)
		return y;
	return deQuyDuoi(x, n - 1, y + pow(x, n));
}

float khuDeQuy(float x, int n)
{
	float tong = x;
	for(int i = 2; i <= n; i++)
	{
		tong += pow(x, i);
	}
	return tong;
}
int main()
{
	float x;
	int n;

	printf("\nNhap x: "); scanf("%f", &x);
	printf("\nNhap n: "); scanf("%d", &n);

	printf("\nDe quy(%f, %d) = %f", x, n, deQuy(x, n));
	printf("\nKhu de quy(%f, %d) = %f", x, n, khuDeQuy(x, n));
	printf("\nDe quy duoi(%f, %d) = %f", x, n, deQuyDuoi(x, n));


	getch();
	return 0;
}

Line 17: error: conio.h: No such file or directory
Line 14: error: expected ';', ',' or ')' before '=' token
In function 'khuDeQuy':
Line 24: error: 'for' loop initial declaration used outside C99 mode