diff --git a/math/tan.c b/math/tan.c
new file mode 100644
index 0000000..3f1b86f
--- /dev/null
+++ b/math/tan.c
@@ -0,0 +1,5 @@
+#include <math.h>
+
+double tan(double x) {
+  return (cos(x - M_PI_2) / cos(x))
+}
\ No newline at end of file