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Difference between Matlab and Wolfram Alpha

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Niklas Kurz
Niklas Kurz on 31 Jul 2020
Edited: Niklas Kurz on 31 Jul 2020
These problems might occure frequently due to the varied syntax. This time it's about:
solve(x^2-8*x+15<=1)
what gives (according to Matlab): pi and 4. That's wrong or not what I ask for. In contrast Wolfram Alpha gives after typing in the same piece of code the correct solution (in my point of view.) Why is that?

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Accepted Answer

Jesús Zambrano
Jesús Zambrano on 31 Jul 2020
Hi Niklas,
You need to set 'ReturnConditions' to true to return any parameters in the solution and conditions on the solution. Therefore,
S = solve(x^2-(8*x)+15<=1, 'ReturnConditions',true);
S.conditions,
ans =
x <= 2^(1/2) + 4 & 4 - 2^(1/2) <= x
in(y, 'real')
which is the solution interval: 2.5858 <= x <= 5.4142

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More Answers (1)

Cam Salzberger
Cam Salzberger on 31 Jul 2020
Hey Niklas,
Technically, both pi and 4 are "correct" results, as if you plug them in, they will fulfill the condition. I think what you are looking for is using the solution = solve(___, "ReturnConditions", true) syntax, as suggested by the solving inequalities example in the documentation.
Also, it would generally be helpful to post what you got from Wolfram as well, so we know what you're expecting.
-Cam

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Niklas Kurz
Niklas Kurz on 31 Jul 2020
Thank you, guys
x <= 2^(1/2) + 4 & 4 - 2^(1/2) <= x
is what I'm looking for.

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