方程（x﹣1）（x﹣2）=1的根是（　　）

用一个平行于棱锥底面的平面截这个棱锥，截得的棱台上、下底面面积比为$1:4$，截去的棱锥的高是$3cm$，则棱台的高是（  ）

    Li Dong is a schoolboy. He's twelve. He lives in Beijing. He gets up at 6:30. He reads English and Chinese in the morning. He has breakfast at 7:00. In the afternoon, he has three classes. He often does the homework after dinner. Li Dong likes playing football. His mother's hobby is planting flowers. His father is interested in collecting stamps. He often shows the nice stamps to Li Dong and his mum. They like the stamps very much.

已知a、b、c是△ABC的三边长，且$\frac{a}{5}$=$\frac{b}{4}$=$\frac{c}{6}$≠0，求：

（1）$\frac{2a+b}{3c}$的值．

（2）若△ABC的周长为90，求各边的长．