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Offline Fog123  
#1 Posted : 06 July 2022 22:26:55(UTC)
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Hi, I have an polynomial equation (x^3 specifically) which I want to get the roots for. The roots are there if I plug in values from WolframAlpha (https://www.wolframalpha.com/input?i2d=true&i=-21.2Power%5Bx%2C3%5D%2B19707.8Power%5Bx%2C2%5D%2B6628342.5x-1125000000), I also checked on excel by changing x=? I found a root of x~128 but SMath won't do it.
On SMath it says no results even after I removed the unit and everything?
Question.jpg

I have used 'solve' function, but always get a "no real roots" message (see file attached).

Can someone help out?
Question.sm (15kb) downloaded 11 time(s).


Many thanks in advance!
Offline Razonar  
#2 Posted : 06 July 2022 22:35:10(UTC)
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Hi. Try solve(f(x),x,0,1500)=1223.
Best regards
Alvaro
Offline overlord  
#3 Posted : 06 July 2022 23:12:34(UTC)
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Try to plot these kind of functions to see what is going on.
Your function is equals to zero around 1200.
Here is your solution with various solvers.

Regards

Question_answer.sm (22kb) downloaded 13 time(s).

2022-07-06_23-10.png
Offline Jean Giraud  
#4 Posted : 07 July 2022 00:49:02(UTC)
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Originally Posted by: Fog123 Go to Quoted Post
I have an polynomial equation ... which I want to get the roots for.

Your polynomial is doctored in yellow.
By inspection, the plot reveals complex roots, i.e: 2 complex roots.
Comment: respect preciously the Lagrange canonical order.
Cheers ... Jean.

Question Doctored.sm (30kb) downloaded 13 time(s).

Offline overlord  
#5 Posted : 07 July 2022 03:58:06(UTC)
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Here is your complete answer by the way.
Good old maple saves the day again.

Regards

2022-07-07_03-54.png
Offline Jean Giraud  
#6 Posted : 07 July 2022 13:59:06(UTC)
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As it looks, if you create cubic at profusion,
the cubic roots solver will be a great companion.
It comes useful in PID techniques.
Polyroots is native in Smath.

Polyroots.PNG
Offline Jean Giraud  
#7 Posted : 07 July 2022 14:38:23(UTC)
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... once your cubic in hand,
you can solve for a specific datum.
Cheers ... Jean.

Solve Datum.PNG
Offline Fog123  
#8 Posted : 11 July 2022 17:24:19(UTC)
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Hi Everyone,

Thank you for your replies they were very helpful. I really appreciate the people who went above and beyond to explain the various ways of getting the roots and even changing the datum. Clap
It proved extremely helpful to plot the function where I quickly realized the difference between my SMath and WolframAlpha results. The 4th term 'x4' or 'c' is supposed to be a negative. Crazy
I though I was doing something wrong with inputting the data into SMath, but it was the function which I was inputting that was wrong.

In my program by changing "Qd" to a (negative) I got the following roots.
Question_finished.JPG

Lesson learned: Quadruple check your functions.

Question_finished.sm (22kb) downloaded 6 time(s).
Offline Jean Giraud  
#9 Posted : 11 July 2022 19:10:43(UTC)
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Good job ! Start be friend with Lagrange canonic.

Question_finished Canonic Lagrange.sm (28kb) downloaded 2 time(s).
Offline Jean Giraud  
#10 Posted : 11 July 2022 19:32:16(UTC)
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... read more in pink.
Cheers ... Jean.

Question_finished Canonic Lagrange.sm (34kb) downloaded 1 time(s).
Offline overlord  
#11 Posted : 11 July 2022 20:05:45(UTC)
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