1.编写程序生成九九乘法表，并将之写入到文本文件exercise7_1.txt中。程序保存为exercise7_1.py。
f = open(r'd:exercise7_1.txt', 'a+')
with open('d:\exercise7_1.txt','a+') as f:
for j in range(1,10):
for i in range(1,j+1):
f.write(str(i)+'*'+str(j)+ '=' + str(i*j) + 't')
f.write('n')
f.close()
2.编写程序，提示用户输入字符串。将所输入的字符串，以及对应字符串的长度写入exercise7_2.txt中。程序保存为exercise7_2.py。
f = open(r'd:exercise7_2.txt', 'a+')
with open('d:\exercise7_2.txt','a+') as f:
s=input("请输入一个字符串：")
f.write("输入的字符串是：" + str(s) +"n"+ "字符串的长度是：" + str(len(s)))
f.close()
3.创建一个exercise7_3.xlsx文件，在C5单元格写入字符串“我喜欢编程”。程序保存为exercise7_3.py。
import openpyxl
workbook=openpyxl.Workbook()
sheet1=workbook.active
sheet1.cell(row=5,column=3).value="我喜欢编程"
workbook.save("d:\exercise7_3.xlsx")
