Lecture 12_Solve_Linear_Equation

Tutorials:

Here is a  very important application of using Matrix in Matlab:  Solve Linear Equations:

Look at the following  equation set:

We need to convert it into this format first:

Then you will get this:

Compare this figure to the last one, you will find out how to extract the coefficients from the equation set and use 'Matrix Multiplication' to represent the linear equation. Remember why we bother ourselves to convert it in this format??? It was explained in last lecture:

This is computer friendly! We need to use this format to get Matlab to calculate it for us!!!!!

Now, the question is HOW?
Let's do it.
If 'A' is the coefficient matrix, X is the variable column, b is the constant column, then the matrix multiplication format can be represented by:
Ax=b

In  linear  Algebra,  the 'Inverse' of a matrix can be represented by A^(-1), you don't need to know how the matrix is inversed now. You only need to know the following:
1) A^(-1) * A = I
2) I * A = A
In whcih I is:

It is making sense that I * A is still A based on the matrix multiplication rules, for example:

Now, one more time:
1) A^(-1) * A = I
2) I * A = A

If we do this:

Then we can get:
x = A^(-1) * b

Which means we can solve the linear equation by A^(-1) * b.
b is given, A^(-1)  is the inverse matrix of A, in Matlab, it is: 'inv(A)'.
Just this simple, you can solve any linear equation set by 'inv(A) * b' in Matlab.

You have a temperature sensor. You know the relation ship between the temperature and the voltage output is:

We need to know c1 and c2 to complete the equation so we can use this equation to predict the temperature based on the voltage we detected.
There is a student did the following measurement:

How can you generalize c1 and c2 based on these measurements?
Let's plug in these values to the equation:

To solve c1 and c2 you only need two equations, why we list so many here?
The reason is, the more equations you have, c1 and c2 can be more accurate or more close to the real value.
This would be redundant if the points were all exactly on a line. Since they are real-life measurements, not ideal, so they are not on a line. If we do this 'over-determined' matrix multiplication in Matlab, Matlab will return the 'best' value for you... (an strong embedded function, you won't see, but Matlab works hard to get you the best value):

Solve c1 and c2 in Matlab.

Now, let's plot it:
V=[2:2:10]; % make an X axis, which is the voltage
T=[88.78 95.79 97.45 107.43 115.85]; % make an Y axis, which is the temperature
plot(V,T,'*'); % plot the data points using * marks.

A=[2 1;4 1;6 1;8 1;10 1];
TT=transpose(T);
xx=A\TT; % Please notice that since A is not square so there is not an invert matrix for A so xx=inv(A)*T won't work.
hold on
line_vs=linspace(0,10,10); % generate a data set for X
line_ts=line_vs*xx(1)+xx(2); % calculate Y
plot(line_vs,line_ts); % plot

No other homework other than these in-class problems. Publish your results in one 'html' file and send it to homeworkflc02@gmail.com by Wednesday.