HDU 2199 Can you solve this equation?（水题吗？！）

#include<stdio.h>
#include<math.h>
int main ()
{
    double a, b, c, y, sum, d;
    int T, x;

    scanf("%d", &T);

    while (T--)
    {
        scanf("%lf", &y);

        x = 0;
        a = 0;
        b = 100;

        d = 8*b*b*b*b + 7*b*b*b + 2*b*b + 3*b + 6;
        c = 8*a*a*a*a + 7*a*a*a + 2*a*a + 3*a + 6;

        if (y >= c && y <= d)
            x = 1;
        if (x)
        {
            while (b-a > 1e-6) ///浮点数判断b是否大于a
            {
                c = (a+b)/2;
                sum = 8*c*c*c*c + 7*c*c*c + 2*c*c + 3*c + 6;

                if (sum > y)
                    b = c-1e-7;
                else
                    a = c+1e-7;
            }

            printf("%.4lf\n", (a+b)/2);
        }

        if (x == 0) printf("No solution!\n");
    }

    return 0;
}