题目

阅读理解 When HarmonyOs, the Chinese self-developedoperating system for Huawei mobile devices, was released on Aug 9, it quicklybecame a hot topic on social media. Many believe it not only represents therise of the country as a tech power, but also shows respect to classicalChinese culture by naming the system Hongmcng in Chinese. Hongmeng is aclassical word from Zhuangzi. In the ancient times of Chinese legend and myths(神话),Hongmengwas used to describe the original state of the universe before matter existed.For HarmonyOS, Hongmeng indicates the developers' aim to make an innovativeop如图，△ABC∽△A1B1C1，且∠A=105°，∠C=30°，则∠B1=    ．