错误提示sqrt没有定义，不知道怎么修改，请指教

#include

#include

#include

#include

int main()

{

char    comments[256];

int    i, j;

double    J=0.0, dx=0.0, dv=0.0;

double    load1=0.0, load2=0.0, load3=0.0, load4=0.0;

double    *coord, *coord0, *coord1, *velocity, *velocity0, *velocity1;

coord = (double *)malloc(1050*sizeof(double));

coord0 = (double *)malloc(1050*sizeof(double));

coord1 = (double *)malloc(1050*sizeof(double));

velocity = (double *)malloc(1050*sizeof(double));

velocity0 = (double *)malloc(1050*sizeof(double));

velocity1 = (double *)malloc(1050*sizeof(double));

FILE    *pp1, *pp2;

pp1=fopen("1129-5000.velS", "r");

pp2=fopen("pengzhuanghousudu.dat", "w");

for( i=0; i<1050; ++i ){

fscanf(pp1, "%lf %lf %lf %lf", &load1, &load2, &load3, &load4);

coord0[i]=load1;

coord1[i]=load2;

coord[i]=sqrt(coord0[i]*coord0[i]+coord1[i]*coord1[i]);

velocity0[i]=load3;

velocity1[i]=load4;

velocity[i]=sqrt(velocity0[i]*velocity0[i]+velocity1[i]*velocity1[i]);

}

for(i=0;i<1050-1;++i){

for(j=i+1;j<1050;++j){

dx=coord[j]-coord[i];

dv=velocity[j]-velocity[i];

J=-1.9*2650.0*2650.0/(2650.0+2650.0)*dv+0.4*1.9*2650.0*2650.0/(2650.0+2650.0)*dv;

if(dx<0.00015){

velocity0[i]+=J/2650.0;

velocity1[i]+=J/2650.0;

velocity0[j]-=J/2650.0;

velocity1[j]-=J/2650.0;

}

}

}

coord0[i]=velocity0[i]*0.000005;

coord1[i]=velocity1[i]*0.000005;

coord0[j]=velocity0[j]*0.000005;

coord1[j]=velocity0[j]*0.000005;

strcpy( comments, "TITLE = \"velocity of sand\"" );   fprintf(pp2, "%s\n", comments);

strcpy( comments, "VARIABLES = \"X\",\"Y\",\"x-velocity\", \"y-velocity\"" );fprintf(pp2, "%s\n", comments);

strcpy( comments, "ZONE I=105,J=10,F=point\n" ); fprintf(pp2, "%s", comments);

for(i=0;i<1050;i++){

fprintf(pp2, "%e    %e     %e    %e\n", coord0[i], coord1[i], velocity0[i], velocity1[i]);

}

fclose(pp1);

fclose(pp2);

}