# How to get 3 variables value from functions?

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Nabilla Dewi Septiani on 15 Nov 2021
clc; clear all; close all; format short g; format compact;
global P A1 B1 C1 A2 B2 C2 Tc1 Pc1 Tc2 Pc2 R g12 g21 g11 g22 a12 x1 x2
P=1.01325*10^5; %Pa
A1=10.0331; B1=2940.46; C1=-35.93;
A2=9.5314; B2=2790.5; C2=-57.15;
Tc1=508.1; Pc1=47.01;
Tc2=523.2; Pc2=38.3;
R=8.314; %Pa.m^3/mol.K
g12=5; g21=5; g11=3; g22=2; a12=1.5;
x1=0:0.1:1;
x2=1-x1;
T=fzero(@NRTLFUN,300); %K
Error using fzero>localFirstFcnEval (line 729)
FZERO cannot continue because user-supplied function_handle ==> NRTLFUN failed with the error below.

Arrays have incompatible sizes for this operation.
Error in fzero (line 286)
fx = localFirstFcnEval(FunFcn,FunFcnIn,x,varargin{:});
plot(T,x1,y1)
function fT=NRTLFUN(T)
global P A1 B1 C1 A2 B2 C2 Tc1 Pc1 Tc2 Pc2 R g12 g21 g11 g22 a12 x1 x2
%Calculating lamda1 and lamda2
tau12=(g12-g22)/(R*T);
tau21=(g21-g11)/(R*T);
G12=exp(-a12*tau21);
G21=exp(-a12*tau12);
lamda1=exp((x2.^2).*((tau21.*((G21./(x1+(x2.*G21))).^2))+((G12.*tau12)./((x2+(x1.*G12)).^2))));
lamda2=exp((x1.^2).*((tau12.*((G12./(x2+(x1.*G12))).^2))+((G21.*tau21)./((x1+(x2.*G21)).^2))));
P1sat=exp(A1-(B1/(T+C1)))*10^5
P2sat=exp(A2-(B2/(T+C2)))*10^5
%fv = (P*v^3)-((R*T+P*b)*v^2)+(a*v)-(a*b);
a1 = (27*R^2*Tc1^2)/(64*Pc1)
b1 = (R*Tc1)/(8*Pc1)
a2 = (27*R^2*Tc2^2)/(64*Pc2)
b2 = (R*Tc2)/(8*Pc2)
fv1 = [P -(R*T+P*b1) a1 -a1*b1];
v1l = min(roots(fv1))
fv2 = [P -(R*T+P*b2) a2 -a2*b2];
v2l = min(roots(fv2))
y1=x1.*lamda1*P1sat*v1l/P
y2=x2.*lamda2*P2sat*v2l/P
fT=y1+y2-1;
end
How to solve this? I want to get T, y1 and y2 values. There is variable in range, which is x1. There are also some functions of T and one function include cubic function
Rik on 15 Nov 2021
Why are you using global variables? That makes debugging harder and is generally bad practice.
What have you tried yourself to find a solution? It looks like your function will error when you input a scalar. Have you confirmed it works as expected on scalar input?

Sahil Jain on 18 Nov 2021
Hi. From my understanding of the question, you want to write a function which outputs the values for "fT", "y1" and "y2". This can be done by changing the function declaration to:
function [fT,y1,y2]=NRTLFUN(T)
The function can then be called by:
[fT, y1, y2] = NRTLFUN(T);
However, this function will not work with "fzero" as "fzero" expects the input function to be scalar-valued. You can read more about function requirements for "fzero" on the documentation page.
Nabilla Dewi Septiani on 6 Dec 2021
Thank you! It is solved now

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