一项调查表明，近年来在中学生中“写情书”现象较常见，一些同学精心制作“情书”。他们有的用印有漂亮图案的信纸，画上一枝“丘比特箭”来表达爱意；也有的用彩笔画上一颗心来表连对对方的情意。即使平时写字歪歪扭扭的同学这时候也写得特别工整。“情书”内容不是流行的爱情歌词，便是表达爱慕的美词佳句。

阅读以上材料，并结合所学知识简要回答下列问题：

    Rick is a good man．He makes birthday cakes in a shop．His cakes are great and have very good prices，so many people buy his cakes．He makes friends with lots of his customers(顾客)．They all like him very much．One evening．he wants to go home，but many of his old customers come in．Rick smiles(微笑)to them，“I am sorry．It is late today．If you need cakes，please come tomorrow．”One of them takes out a nice cake and says to him，“You make so many great cakes for us．This birthday cake is for you．Happy birthday, Rick．”

 在学习了滑动摩擦力知识后，小明和小华想用所学的知识进一步探究运动鞋的鞋底防滑效果，他们各自带来了洗干净的运动鞋，又准备了一张练习立定跳远用的橡胶垫，一个弹簧测力计和细线．

若$\frac{1}{a}-\frac{1}{b}=4$，则$\frac{a-2ab-b}{2a+7ab-2b}$的值是____