Complex numbers appearing from no where.

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Tonny 2013-10-31

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https://ww2.mathworks.cn/matlabcentral/answers/104479-complex-numbers-appearing-from-no-where

评论： Walter Roberson 2013-10-31

Hi guys,

My formula suppose to plot a parabolic surface

0.666 = sqrt(X^2+ Y^2+ Z^2) - sqrt((X-1)^2+ Y^2+ Z^2))

So i solve for Y using the Matlab solver and then plot it with a meshgrid. At the moment i plot only with the real value.

x = 0.5 : 0.1 : 10; z = -10 : 0.1 : 0; [X,Z] = meshgrid(x,z);

I'm expecting a cone type plot but somehow there this extra plane like thing below.

I tried removing the real(Y) yet nothing is being plotted. But i realize that the plane like thing is actually created due to complex number being taken only the real side thus Y~=0

Can i ask is it because i use meshgrid and it forces values of X and Z into the equation?

I tried doing Y(imag(Y) ~= 0) = NaN; But result is still incomplete.

Help!

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Walter Roberson 2013-10-31

What formula are you using for Y ? Are you taking into account that there are two solutions, negatives of each other, so you need to include both of them to get the proper output ?

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