adding a simpler solution to 'sum over last two axes' exercise'

100 Numpy exercises.md

@@ -693,6 +693,11 @@ print(np.unique(I))
 
 ```python
 A = np.random.randint(0,10,(3,4,3,4))
+# solution by passing a tuple of axes
+sum = A.sum(axis=(-2,-1))
+print(sum)
+# solution by flattening the last two dimensions into one
+# (useful for functions that don't accept tuples for axis argument)
 sum = A.reshape(A.shape[:-2] + (-1,)).sum(axis=-1)
 print(sum)
 ```