diff --git a/costfunctions.py b/costfunctions.py
index 38a92a9..589f13d 100644
--- a/costfunctions.py
+++ b/costfunctions.py
@@ -43,4 +43,7 @@ def sigmoid(z):
     return 1.0/(1.0+np.exp(-z))
 
 def sigmoid_prime(z):
-    return sigmoid(z)*(1-sigmoid(z))
\ No newline at end of file
+    return sigmoid(z)*(1-sigmoid(z))
+
+def quadratic_cost(a, y):
+        return 0.5*np.linalg.norm(a-y)**2