fixed comments

MathTests.cs

@@ -49,8 +49,8 @@ public void SolveQaudratic_WithPositive_C_Argument_Should_ReturnOneSameRoot()
         var result = QuadraticHelper.Solve(a, b, c);
 
         // Assert
-        result.Length.ShouldBe(2);
-        result[1].ShouldBe(result[0]);
+        result.Length.ShouldBe(1);
+        result[0].ShouldBe(result[0]);
     }
 
     [Fact]
@@ -74,4 +74,24 @@ public void SolveQaudratic_WithPositive_A_Argument_Should_NotBeNull()
         }
 
     }
+
+    [Theory]
+    [Trait("Category", "Unit")]
+    [InlineData(double.NaN, 2, 3, "Argument a is incorrect")]
+    [InlineData(4, double.NaN, 3, "Argument b is incorrect")]
+    [InlineData(5, 2, double.NaN, "Argument c is incorrect")]
+    public void SolveQaudratic_With_A_NaN_Argument_Should_ThrowException(double a, double b, double c, string messageError)
+    {
+
+        // Act
+        try
+        {
+            var result = QuadraticHelper.Solve(a, b, c);
+        }
+        catch (Exception e)
+        {
+            // Assert
+            e.Message.ShouldBe(messageError);
+        }
+    }
 }

QuadraticHelper.cs

@@ -4,7 +4,9 @@ public static class QuadraticHelper
     {
         public static double[] SolveIncompleteQuadraticEquation(double b)
         {
-            if (b < 0)
+            ValidateDoubleParameter(b, "Argument b is incorrect");
+
+            if (b.CompareTo(0) < 0)
                 return new double[0];
 
             var res = Math.Sqrt(b);
@@ -13,19 +15,39 @@ public static double[] SolveIncompleteQuadraticEquation(double b)
 
         public static double[] Solve(double a, double b, double c)
         {
-            if (a == default)
-                throw new ArgumentException("Argument a can not be 0");
-
-
+
+            ParametersValidation(a, b, c);
+
             double d = b * b - 4 * a * c;
 
-            if (d.EqualsExact(0))
-                throw new ArgumentException("Diskriminant is 0");
+            if (d.CompareTo(0) < 0)
+                return new double[0];
+            else if (d.CompareTo(0) == 0)
+            {
+                var x1 = (-b) / 2 * a;
+                return new double[] { x1 };
+            } else
+            {
+                var x1 = (-b + Math.Sqrt(d)) / 2 * a;
+                var x2 = (-b - Math.Sqrt(d)) / 2 * a;
+                return new double[] { x1, x2 };
+            }
+        }
 
-            var x1 = (-b + Math.Sqrt(d)) / 2 * a;
-            var x2 = (-b - Math.Sqrt(d)) / 2 * a;
+        private static void ParametersValidation(double a, double b, double c)
+        {
+            if (a.EqualsExact(0))
+                throw new ArgumentException("Argument a can not be 0");
 
-            return new double[] { x1, x2 };
+            ValidateDoubleParameter(a, "Argument a is incorrect");
+            ValidateDoubleParameter(b, "Argument b is incorrect");
+            ValidateDoubleParameter(c, "Argument c is incorrect");
+        }
+
+        private static void ValidateDoubleParameter(double val, string errorMessage)
+        {
+            if (!double.IsFinite(val) || double.IsNaN(val))
+                throw new ArgumentException(errorMessage);
         }
     }
 }