hdu 2199 Can you solve this equation?

求导后发现，f ‘ 恒大于0，用二分法求即可，剪除条件：f(100)<y || f(0)>y。最后输出mid，结果WA，改为输出(left+right)/2通过。后来明白：当left=right时会跳出循环，而mid是不正确的。

 

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

double cal(double x){
return 8*pow(x, 4)+7*pow(x,3)+2*pow(x,2)+3*x+6;
}

int main()
{
int t;
double y,left,right,mid;
scanf("%d",&t);
while(t--){
scanf("%lf",&y);
if(cal(100)<y||cal(0)>y){
printf("No solution!\n");
continue;
}
left=0;
right=100;
while(right-left>1e-6){
mid=(left+right)/2;
double ans=cal(mid);
if(ans<y){
left=mid+1e-6;
}
if(ans>y){
right=mid-1e-6;
}
}
printf("%.4lf\n",(left+right)/2);

}

return 0;
}