diff --git a/src/numerical/calculus.py b/src/numerical/calculus.py
index a0e2226..3e54939 100644
--- a/src/numerical/calculus.py
+++ b/src/numerical/calculus.py
@@ -8,9 +8,10 @@ def numerical_integration_rectangle(
         a, b = b, a
     h = (b - a) / n
     result = 0.0
+    x = a
     for i in range(n):
-        x = a + i * h
         result += f(x)
+        x += h
     return result * h