“脱缰之马”﹣﹣癌细胞由“叛变”的正常细胞衍生而来，是产生癌症的病源，下列关于癌细胞的说法，正确的是（  ）

    Hutong is a kind of ancient typical city 5lrccl in Beijing.____ (surround) the Forbidden City, many were built during the Yuan, Ming and Qing ____ (dynasty). In order to establish the strongest power for emperors, the royal family planned the city and arranged the residential areas (住宅区). The Forbidden City- ____ center of the Beijing city, was the royal palace.

    At the end of the Qing dynasty, after ____ (close) China came under the influence from abroad, many old hutongs lost____ (they) former neat (灵巧的) arrangement.

    After the founding of the People's Republic of China in 1949, the conditions of hutong ____ (improve). But in recent years, the houses in many hutongs have been pulled ____ and then replaced by modem buildings. And the hutong today is fading into the shade for both tourists____ local people.

    However, in the urban district of Beijing, hutongs still occupy one ____ (three) of the total area, providing housing for half population, so many hutongs ____ (survive) these years. In this respect, we see Beijing as an ancient yet modem city.

先化简，再求值：

a（a²b²﹣ab）﹣b（a²﹣a³b）÷2a²b，其中a=﹣ $\frac{1}{2}$ ，b= $\frac{1}{3}$ ．