feed_dict ：喂数据
tf.placeholder：喂参数给损失函数
import numpy as np
import tensorflow as tf
coefficients = np.array([[1.],[-10.],[20.]])
w = tf.Variable(0,dtype=tf.float32)
x = tf.placeholder(tf.float32,shape=[3,1])
cost = x[0,0]*w**2 + x[1,0]*w + x[2,0]
train = tf.train.GradientDescentOptimizer(0.01).minimize(cost)
init = tf.global_variables_initializer()
session = tf.Session()
session.run(init)
for i in range(100):
session.run(train,feed_dict={x:coefficients})
print(session.run(w))
