明清(鸦片战争前)时期,中国在传统农耕文明的轨道上发展到一个新的阶段,下列对这一时期历史阶段特征最恰当的概括是( )
A:帝国的繁盛
B:统一多民族国家空前巩固与发展
C:近代的曙光
D:帝国的繁盛与近代前夜的危机
阅读短文,回答问题。
It's eight o'clock. The children go to school by car every day. But they are walking to school today. It's ten o'clock. Mrs. Green usually(通常) stays at home in the morning. But she is going to the shops this morning. It's nine o'clock. Mr. Green usually reads his newspaper at night(晚上). But he is reading an interesting(有趣的) book this evening.
读下图,a、b、c分别表示0~14岁、15~64岁、65岁以上三种年龄人数所占总人口比重.据此回答下题.
小明同学遇到下列问题:
解方程组 {2x+3y4+2x−3y3=72x+3y3+2x−3y2=8 ,他发现如果直接用代入消元法或加减消元法求解,运算量比较大,也容易出错.如果把方程组中的(2x+3y)看作一个数,把(2x﹣3y)看作一个数,通过换元,可以解决问题.以下是他的解题过程:
令m=2x+3y,n=2x﹣3y.
这时原方程组化为 {m4+n3=7m3+n2=8. 解得 {m=60n=−24
把 {m=60n=−24 代入m=2x+3y,n=2x﹣3y.
得 {2x+3y=602x−3y=−24 解得 {x=9y=14
所以,原方程组的解为 {x=9y=14
请你参考小明同学的做法,解决下面的问题: