>>342
とりあえず2分法
#include <stdio.h>
#include <math.h>
#define E 1.0e-11

double f(double x) { return 1 / x - tan(x); }

int main(void)
{
double x0 = 0.5, x1 = 1.5, m;
int i = 0;

do {
printf("%4d: f(%f) = %f, f(%f) = %f\n", ++i, x0, f(x0), x1, f(x1));
m = (x0 + x1) / 2;
if (f(x0) * f(m) >= 0) x0 = m;
else if (f(x1) * f(m) >= 0) x1 = m;
} while (fabs(f(x0) - f(x1)) > E);

printf("ans: %20.16f\n", x0);
return 0;
}