diff --git a/C/quadratic_equations.c b/C/quadratic_equations.c
new file mode 100644
index 0000000..c0ce76c
--- /dev/null
+++ b/C/quadratic_equations.c
@@ -0,0 +1,34 @@
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <math.h>
+     
+int main()
+{
+  double d, x;
+  double a = 1;
+  double b = 2;
+  double c = 1;
+     
+  if (a != 0) {
+    b /= a;  c /= a;     // aで割ってx^2 + bx + c = 0$の形にする
+    if (c != 0) {
+    	b /= 2;          // x^2 + 2b'x + c = 0
+    	d = b * b - c;   // 判別式(D/4)を計算
+    	if (d > 0) {
+    		if (b > 0) x = -b - sqrt(d);
+    		else       x = -b + sqrt(d);
+    		printf("x = %g, %g\n", x, c / x);
+    	} 
+    	else if (d < 0)
+    		printf("x = %f +- %f\n", -b, sqrt(-d));
+    	else
+    		printf("x = %f\n", -b);
+    	} 
+    else printf("x = %f, 0\n", -b);
+  } 
+  else if (b != 0) printf("x = %f\n", -c / b);
+  else if (c != 0) printf("解なし\n");
+  else printf("不定\n");
+  return 0;
+}