How can i call the value of another variable defined or solved above for solving another equation?

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zy
zy on 23 Jul 2014
Commented: zy on 24 Jul 2014
Right now, I have a 6-order polynomial expression, it has only one variable.
I want to be able to call values of some variable defined or obtained above, and solve the polynomial equation for different values of the expression.
I used the following code, but it does not work.
p(1)=1,p(2)=2,p(3)=3,p(4)=4,p(5)=5,p(6)=6,p(7)=7;
f(1)=1,f(2)=2,f(3)=3,f(4)=4,f(5)=5,f(6)=6,f(7)=7;
solve('p(1)*(x.^6)+p(2)*(x.^5)+p(3)*(x.^4)+p(4)*(x.^3)+p(5)*(x.^2)+p(6)*x+ p(7)=f(1)');
what mistake have I made? or in what ways can achieve this purpose?
Thank you.

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Accepted Answer

Roger Stafford
Roger Stafford on 23 Jul 2014
Edited: Roger Stafford on 23 Jul 2014
Instead of using 'solve' try using 'roots' for numerical solutions. It will be much faster.
Also, 'solve' will in general be unable to find symbolic solutions for sixth order polynomials.
  1 Comment
zy
zy on 24 Jul 2014
Thank you very much, 'roots' works.
Since it is a 6 order polynomial equation, there may be up to 6 roots including real roots and complex roots.
Can I do something to filter or pick up only real roots?

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More Answers (1)

Matz Johansson Bergström
Matz Johansson Bergström on 23 Jul 2014
Edited: Matz Johansson Bergström on 23 Jul 2014
Try
syms x
p = 1:7;
f = 1:7;
solve(p(1)*x^6+p(2)*x^5+p(3)*x^4+p(4)*x^3+p(5)*x^2+p(6)*x+p(7)==f(1), x)

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