error using Solve with inequality

Hi all,

I am facing a strange problem with using Solve with inequality.

Could you please help?

Thanks

[Torsten]

Why strange ?

Isn't

4^2-4*4+3 > 0 and 0^2-4*0+3 > 0

as well as

e^2-4*e+3 < 0 and 2^2-4*2+3 < 0

?

[Shan Chu]

Hi,

I thought that the solve function would give me the intervals where the inequality is valid.

Instead, it gave me 2 specific values inside those intervals. (of course they are correct).

I saw the document about solve in matlab website. Why shoudn't it give me the intervals like in the website. I included the pics below

Thanks

Answer 1

[John D'Errico]

You did not follow the example carefully.

syms x
>> S = solve(x^2 - 4*x + 3 < 0,'returnconditions',true)
S = 
  struct with fields:

             x: [2×1 sym]
    parameters: [1×2 sym]
    conditions: [2×1 sym]
>> S.conditions
ans =
 1 < x & x < 3
 in(y, 'real')

Note the use of returnconditions. Without that, it gives a solution, but not the interval that you desire. So if I drop that property, we get a solution. It is valid, as you say. But just a solution.

S = solve(x^2 - 4*x + 3 < 0)
S =
 exp(1)
      2

[Shan Chu]

Oh I see it now. Thank John.