python broadcast 和 numpy.sum()函数

import numpy as np
a = np.random.random_sample((3,1,3))
b  = np.random.random_sample((2,3))
c = a-b
c = np.square(c)
c = np.sum(c,axis=2)
c= np.sqrt(c)
a1 = a[1,:,:]
b1 = b[1,:]
print(a1,'5')
print(b1,'6')
print(np.square(a1-b1).shape)
print(np