MATLAB第二次作业易错点+2

MATLAB 第二次作业易错点

PART A

2. Using plot to display the following voltage with appropriate line type, title and labels. Also present the graph with suitable ranges of axis.

ft e t v t π2sin 220)(5-=

where f =50 Hz.

In addition, on the same graph, draw the envelope of the oscillation and add legends.

共同错误：没有理解题意，本题不仅要画出所给的函数的曲线图，还需要画出曲线外围的包络线。包络线画法提示： ()v t 函数可以分为1()v t =5220t e -与2()v t =sin 2ft π两个函数的乘积，v 值的变化与这两个函数同时有关，可以观察包络线是1()v t 的曲线，而两条包络线恰好基于0v =对称。

6．Load 2 of your photos into Matlab WorkSpace using imread. a) Change brightness locally or globally. b) Overlap them to produce a new photo. c) write a new photo into a file using imwrite. (Note: lower version of Matlab such as 6.5 is not allowed to do some direct image operations.)

共同错误：a,b 部分都没有什么问题，c 部分很多同学没明白是什么意思，老师的意思应该是使用imwrite 命令将读入的图片进行不同格式处理后存入新的文件中，例如： imgrgb = imread ('flower.jpg ');

imwrite(imgrgb, 'flower.bmp ', 'bmp'); % jpg 格式转换为bmp 格式

具体可查看百度百科。

7. For linear simultaneous equations

M

N MN M M N N N N b x a x a x a b x a x a x a b x a x a x a =+++=

+++

=+++

..................

(2212)

2222121112112111

the equation coefficients:

A=[ 1 -1 4 3

-5 4 5 -6

0 7 -8 9

-1 3 -2 6]; (M=N)

a) Find the determinant of A,

b) Find the inverse of A and check for matrix singularity,

c) If B=[ 5; 1; -2; 3], find the unknown x in the equation 。

共同错误：b 题很多同学不明白如何检查矩阵是否奇异，其实在线性代数课上，老师有讲过的哦，一个函数就可以判断了，()cond A 如果非常大，则A 是奇异的。如果是一个具体的小些的数，则A 是非奇异的，如本题算出结果是16.4，是非奇异的。

8. For linear simultaneous equations,

M N MN M M N N N N b x a x a x a b x a x a x a b x a x a x a =+++=

+++

=+++

......

............

(2212)

2222121112112111

M>N, the equation coefficients:

A=[ 1 -1 4 3

-5 4 5 -6

0 7 -8 9

-1 3 -2 6

1 -

2 5 3

0 4 7 -3];

a) Find the determinant of A’*A,

b) Find the inverse of A’*A and check for matrix singularity,

c) If B=[ 5; 1; -2; 3; 4; 0], find the solution of the equation.

共同错误：同上题，而检查矩阵是否奇异时，应对('*)cond A A 进行判断。

9.Data of 10 records are shown below

y=[3.5 4.3 3.7 5.4 6.6 7.3 8.7 8.8 9.4 9.0 10.0 12.0 11.3 9.9 13.3],

Use polyfit with different orders (from 1 to 3) of polynomials to find a curve of best fit. Check the total distance between the fitted curve z and records defined by

()2/12⎪⎪⎭⎫ ⎝⎛-=∑i i i y z s .

共同错误：本题不少同学只给出了哪一种拟合方式最优，但是题目要求，不仅要画出拟合曲线图，而且要根据

()2/12⎪⎪⎭⎫ ⎝⎛-=∑i i i y z s 计算出每一种拟合的误差。并根据误差值给出最

佳的拟合方式。 PART B

2. A three phase induction motor characteristic is given in terms of mechanical shaft output torque M T (NM Newton-meter) as a function of rotational speed ω (rad/s radian per second). This is approximated by 3 piece-wise linear equations as follows: