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#include <cassert>
double ddivid(double (*foo)(double),double x1,double x2,double err)
{
assert(x2>x1);
assert(foo(x2)*foo(x1)<0);
while(x2-x1>err)
{
double x=(x2+x1)/2.;
double y=foo(x);
double y1=foo(x1);
double y2=foo(x2);
if(y1*y<0)
{
x2=x;
}
else if(y2*y<0)
{
x1=x;
}
else
{
assert(0);
}
}
return (x1+x2)/2.;
}
double ddivid(double (*foo)(double),double z,double x1,double x2,double err)
{
assert(x2>x1);
assert((foo(x2)-z)*(foo(x1)-z)<0);
while(x2-x1>err)
{
double x=(x2+x1)/2.;
double y=foo(x)-z;
double y1=foo(x1)-z;
double y2=foo(x2)-z;
if(y1*y<0)
{
x2=x;
}
else if(y2*y<0)
{
x1=x;
}
else
{
assert(0);
}
}
return (x1+x2)/2.;
}
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