diff --git a/tests/test_operation.py b/tests/test_operation.py
index 07e3ec7..b512ff6 100644
--- a/tests/test_operation.py
+++ b/tests/test_operation.py
@@ -3,9 +3,17 @@
 def test_add():
     assert add(2,3)==5
     assert add(-1,1)==0
+    assert add(0,0)==0  
+    assert add(-1,-1)==-2
+
     
 def test_sub():
     assert sub(5,3)==2
     assert sub(4,3)==1
     assert sub(3,3)==0
-    assert sub(2,3)==-1
\ No newline at end of file
+    assert sub(2,3)==-1
+    assert sub(1,3)==-2
+    assert sub(0,3)==-3
+    assert sub(-1,3)==-4
+
+    assert sub(2,3)==-1