help with equation substituting

2016 3 10
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更新时间：2016 3 12
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hi i want to substitute in an equation that has 2 variables (q) and (pwf), given that the user has to input several values for (pwf) to substitute into the equation and solve for (q)
Thank you

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Walter Roberson
Walter Roberson 2016-3-10

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Example:
new_pwf = rand();
new_equation = subs(TheEquation, pwf, new_pwf);
sol = solve(new_equation, q)

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mohamed
mohamed 2016-3-10
ok this is what i did
clc
clear
new_pwf=[0:500:2500]
new_equation=subs('q=((1-(0.2*(pwf/pr)))-(0.8*(pwf/pr)^2))*(1067)',pwf,new_pwf);
sol=solve(new_equation,q)
but it gave me an error
Undefined function or variable 'pwf'.
Error in Untitled2 (line 4) new_equation=subs('q=((1-(0.2*(pwf/pr)))-(0.8*(pwf/pr)^2))*(1067)',pwf,new_pwf);
Walter Roberson
Walter Roberson 2016-3-10
syms pwf pr q
expression = ((1-(0.2*(pwf/pr)))-(0.8*(pwf/pr)^2))*(1067);
new_expression = subs(expression, pfw, 0:500:2500);
There is no point solving for a variable (q) that appears isolated on one side of a comparison.
Solving a vector of expressions can only work when the number of elements in the vector is the same as the number of variables indicated to solve for. With 5 values for the new pwf you would need to be solving for 5 variables.
mohamed
mohamed 2016-3-11
编辑：mohamed 2016-3-11
ty very much you really helped me if i may ask another question whne i substitute into an equation this is the result
q =
1.0e+03 *
1.0670 0.9902 0.8451 0.6317 0.3500 0
i need the results to be
1067 990 845 632 350 0 i dont want them multiplyed by 1.0e+03 and i tried the double() code and it didnt work thank you

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