A:缺少图例
B:不知道每个月的销量
C:不能够正确反映出销量情况
D:看不出哪个月的销量最多
对于函数y=f(x),若存在定义域D内某个区间a,b,使得y=f(x)在a,b上的值域也为a,b,则称函数y=f(x)在定义域D上封闭,如果函数f(x)=﹣4x1+x在R上封闭,则b﹣a=____
— He's never advised to achieve a balance between his job and hobbies, ________? — _______. His parents and teachers often offer him advice.