implement minor for matrix3x3

rtchallenge/src/matrices.rs

@@ -81,6 +81,20 @@ impl Matrix3x3 {
         let m = [[rows[0][0], rows[0][1]], [rows[1][0], rows[1][1]]];
         Matrix2x2 { m }
     }
+
+    ///
+    /// # Examples
+    /// ```
+    /// use rtchallenge::matrices::{Matrix2x2, Matrix3x3};
+    ///
+    /// let a = Matrix3x3::new([3., 5., 0.], [2., -1., -7.], [6., -1., 5.]);
+    /// let b = a.submatrix(1, 0);
+    /// assert_eq!(b.determinant(), 25.0);
+    /// assert_eq!(b.determinant(), a.minor(1, 0));
+    /// ```
+    pub fn minor(&self, row: usize, col: usize) -> f32 {
+        self.submatrix(row, col).determinant()
+    }
 }
 impl Index<(usize, usize)> for Matrix3x3 {
     type Output = f32;