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# i have a question that works backwards

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Omar Almahallawy on 28 Apr 2019
Closed: MATLAB Answer Bot on 20 Aug 2021
I have a set of values for F and a set of values for D.
When F and D are divided together they give a ratio that I have the answers to in variable X
so
X=F./(D.^5)
ex.
F =[
0.0026 0.0026 0.0026 0.0026 0.0027 0.0027 0.0027 0.0028 0.0028 0.0028]
D =[
0.9652 1.0160 1.0668 1.1176 1.1684 1.2192 1.3208 1.4224 1.5240 1.6256]
X=[0.0024 0.0019 0.0015 0.0012 0.0010 0.0007 0.0005 0.0003 0.0002]
I want code that tells me for which F and which D give me the known answer of X
dpb on 29 Apr 2019
That's a trivial Q? as posed; you calculated X_i from F,D_i so the answer is simply "i" for the set of calculated values.
IF you somehow generate the identically-computed X from some other location, then that Xprime value would match one of the originals; to find which one would be simply
indx=find(X==Xprime);
BUT as was noted in the previous comment, that exact lookup will fail almost certainly owing to floating point rounding and precision issues; perhaps ismember could help resolve that particular problem but somehow I don't think you've yet described what you're after sufficiently for us to understand what the objective is here.

Jos (10584) on 29 Apr 2019
% X is known
F = X
D = ones(size(F))
% F ./ (D.^5) equals X
or is this to simply thought by me ;-)