Code feedback for ibaesuyeon

hello.c

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

// Consistent code styling is important.
// This is my preference, but I uses "2 spaces" for indentation
// (as opposed to using "tab") because github renders tab with 8 spaces.
// You can configure your code editor to use spaces instead of tab,
// even when you're pressing a tab.

// Recursive style factorial function. Returns x!.
long long factorial(int x) {
  // "LL" in 1LL is the literal suffix to use the long long value.
  // See https://boycoding.tistory.com/155.
  return (x <= 1) ? 1LL : (long long)(x) * factorial(x - 1);
}

// Recursive style power function. Returns x^exp.
double power(double x, int exp) {
  return (exp <= 0) ? 1.0 : x * power(x, exp - 1);
}

// sin(x) taylor approximation with naive implementation.
double sin_taylor_naive(double rad, int max_order) {
  // fmod is a modular operation for floating point value (thus the name fmod).
  // Clip values outside -pi <= rad <= pi.
  rad = fmod(rad, M_PI);
  double result = 0.0;
  for (int k = 1; k <= max_order; k += 2) {
    // "Sign" is a computer science term whether the value is positive or negative.
    double sign = (k % 4 == 3) ? -1 : 1;
    // Large value divided by large value is numerically unstable.
    result += sign * power(rad, k) / factorial(k);
    // Un-comment below to see how coefficients goes Inf or NaN when k is big.
    // printf("DEBUG: k=%d sign=%1.0lf power=%lf coef=%lf\n", 
    //         k, sign, power(rad, k), 1.0 / factorial(k));
  }
  return result;
}

// sin(x) taylor approximation with FMA (Fused multiplication-addition)
// implementation.
double sin_taylor_fma(double rad, int max_order) {
  rad = fmod(rad, M_PI);  // Clip values outside -pi <= rad <= pi.
  double result = 1.0;

  // Taylor expansion for sin(x) has only odd-order terms.
  if (max_order % 2 == 0) max_order -= 1;

  for (int k = max_order; k > 1; k -= 2) {
    result = 1.0 - rad * rad / ((k - 1) * k) * result;
  }
  result *= rad;
  return result;
}

int main() {
  double degree;
  int max_taylor_order;
  // We use "Enter" instead of "Inject" on interactive command line program.
  printf("Enter the degree of sine: ");
  scanf("%lf", &degree);  // Note
  printf("Enter the maximum taylor expansion order: ");
  scanf("%d", &max_taylor_order);
  double rad = degree * M_PI / 180.0;  // We have M_PI constant already defined
                                       // instead of using 3.14159265...

  // Print double (lf) with 15 decial digits, which is full decimal precision.
  printf("sin(x) = %0.15lf\n", sin(rad));
  printf("sin_taylor_navie(x) = %0.15lf\n",
         sin_taylor_naive(rad, max_taylor_order));
  printf("sin_taylor_fma(x) = %0.15lf\n",
         sin_taylor_fma(rad, max_taylor_order));

  return 0;
}