🧪 Add tests for Cas.evaluate

test/cas_test.dart

@@ -0,0 +1,63 @@
+import 'package:functionx/src/cas/cas.dart';
+import 'package:test/test.dart';
+
+void main() {
+  group('Cas', () {
+    group('evaluate', () {
+      test('differentiates single variable: d/dx(x^2)', () {
+        // math_expressions derivative logic produces this unsimplified form
+        expect(Cas.evaluate('d/dx(x^2)'), equals('((x^2.0) * (2.0 * (1.0 / x)))'));
+      });
+
+      test('differentiates trig function: d/dx(sin(x))', () {
+        // sin(x) derives to cos(x) * 1, which simplifies to cos(x)
+        expect(Cas.evaluate('d/dx(sin(x))'), equals('cos(x)'));
+      });
+
+      test('differentiates constant: d/dx(5)', () {
+        expect(Cas.evaluate('d/dx(5)'), equals('0.0'));
+      });
+
+      test('differentiates with different variable: d/dt(t^2)', () {
+        expect(Cas.evaluate('d/dt(t^2)'), equals('((t^2.0) * (2.0 * (1.0 / t)))'));
+      });
+
+      test('integrates variable: int(x)dx', () {
+        expect(Cas.evaluate('int(x)dx'), equals('0.5*x^2'));
+      });
+
+      test('integrates variable (implicit dx): int(x)', () {
+        expect(Cas.evaluate('int(x)'), equals('0.5*x^2'));
+      });
+
+      test('integrates trig function: int(cos(x))dx', () {
+        expect(Cas.evaluate('int(cos(x))dx'), equals('sin(x)'));
+      });
+
+      test('integrates constant: int(5)dx', () {
+        expect(Cas.evaluate('int(5)dx'), equals('5.0*x'));
+      });
+
+      test('simplifies expression: x + x', () {
+        // simplification logic does not combine like terms in this version
+        expect(Cas.evaluate('x + x'), equals('(x + x)'));
+      });
+
+      test('evaluates equation: d/dx(x^2) = 2*x', () {
+        expect(Cas.evaluate('d/dx(x^2) = 2*x'), equals('((x^2.0) * (2.0 * (1.0 / x))) = (2.0 * x)'));
+      });
+
+      test('throws on invalid derivative format', () {
+        expect(() => Cas.evaluate('d/dx'), throwsFormatException);
+      });
+
+      test('throws on invalid integral format', () {
+        expect(() => Cas.evaluate('int(x'), throwsFormatException);
+      });
+
+      test('throws on unsupported integration', () {
+         expect(() => Cas.evaluate('int(tan(x))dx'), throwsFormatException);
+      });
+    });
+  });
+}