—Must I do my homework now?
—No, you . You may have a rest.
已知向量a→=1,2,向量b→=x,-2,且a→∥b→,则实数x等于( )
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.