Plots of functions involving quadratic denomonator

Question

Timothy 2011-4-3

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https://ww2.mathworks.cn/matlabcentral/answers/4664-plots-of-functions-involving-quadratic-denomonator

I am having great trouble with a plot I think should be very simple. I want to plot the following:

x=[0:1:100]; y=6-400/(100+x.^2);

My first stab at google lead me to the x. trick, but strangely it seems to fail if I have a quadratic expression in the denominator. I assume this is a syntax problem. If I try to enter the second line above, I get the following error:

??? Error using ==> mldivide Matrix dimensions must agree.

I understand that x is a matrix, but how come it works when this term is in the numerator instead?

I appreciate any help you can give on this. Thanks for reading.

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Answer 1

Jarrod Rivituso 2011-4-4

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在 MATLAB Online 中打开

The "x trick" you are using is actually the element-wise exponentiation. MATLAB is inherently a matrix language (MATrix LABoratory) and by default it wants to do matrix computations.

So, the reason for your error is that you need to be using element-wise division too. Again, use the . to make it element-wise.

>> y = 6 - 400./(100+x.^2)

The reason it worked when x was in the numerator is because dividing or multiplying by a scalar is a special case that MATLAB makes an exception for.