Unable to find explicit solution to a very simple equation
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Teemu Hämäläinen
on 15 Sep 2021
Commented: Teemu Hämäläinen
on 15 Sep 2021
So my code is
syms x
solve(-3*x^4+12*x^2 > 0,x)
And for some reason matlab cant answer this. I have tried everything but nothing has worked so far
the answer should be x=0 and x=+-2, but instead i get
Warning: Unable to find explicit solution. For options, see help.
ans =
Empty sym: 0-by-1
What am i doing wrong? I am losing my mind cause I know this is a very simple task
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Accepted Answer
Paul
on 15 Sep 2021
Edited: Paul
on 15 Sep 2021
The equation in the question is an inequality, but the supposed solution posed in the question is for an equality:
syms x real
f(x) = -3*x^4 + 12*x^2;
solve(f(x)==0,x)
Plot f(x)
fplot(f(x),[-2 2])
So that gives us an idea of what the solution should be for f(x) > 0.
assume(x,'real') % otherwise the solver will consider x as complex
sol = solve(f(x)>0,'ReturnConditions',true);
sol.x
sol.parameters
sol.conditions
Which looks like the expected solution based on the graph.
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