表达式 '+'.join（['1','2','3']) 的值为6。（）

已知一沿x正方向传播的平面余弦波，时的波形如图所示，且周期T为2s。

(1)写出O点的振动表达式;

(2)写出该波的波动表达式;

(3)写出A点的振动表达式;

(4)写出A点离O点的距离。

设有N个粒子的系统，其速率分布如题5-9图所示求

(1)分布函数f(v)的表达式;

(2)a与V_o之间的关系;

(3)速率在1.5V_o到2.0V_o之间的粒子数;

(4)粒子的平均速率;

(5)0.5V_o到1V_o区间内粒子平均速率。

设f(x)=g₁(x).g₂(x)，其中g₁(x), g₂(x)在(-∞，+∞)内满足条件且.

(1)求f(x)所满足的一阶微分方程

(2)求出f(x)的表达式

数字控制系统结构图如图8-14所示，采样周期T=1s。

(1)试求未校正系统的闭环极点，并判断其稳定性。

(2)x_t(t)=t时，按最少拍设计，求D(z)表达式，并求X₀(z)的级数展开式。

(1)计算脉振磁动势基波和3、5、7等次谐波的振幅，并写出各相基波脉振磁动势的表达式

(2)当B相电流为最大值时，写出各相基波磁动势的表达式

(3)计算三相合成磁动势基波及5、7、11次谐波的幅值，并说明各次谐波的转向、极对数和转速

(4)写出三相合成磁动势的基波及5、7、11次谐波的表达式

(5)分析基波和5、7、11次谐波的绕组系数值，说明采用短距和分布绕组对磁动势波形有什么影响

β-胡萝卜素(β-carotene)的结构式如下：

(1)若将它的π电子近似地按一维箱中粒子处理，试估算势箱的长度。

(2)写出与其HOMO和LUMO对应的能级表达式。

(3)计算将电子从HOMO激发到LUMO所需的能量及相应的光的波长。

(4)β-胡萝卜素显红色，是否可用上述一维势箱模型说明？若不能用此模型解释，可能的因素是什么？

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求函数f(x)(0＜x＜1)的表达式,其中