## How to create a mathematical expression which contains delays using Symbolic Math Toolbox?

Asked by Raja Vardhan Reddy Kothakapu

### Raja Vardhan Reddy Kothakapu (view profile)

on 4 Jun 2019
Latest activity Edited by Raja Vardhan Reddy Kothakapu

### Raja Vardhan Reddy Kothakapu (view profile)

on 5 Jun 2019
Hi everyone,
I'm trying to create a mathematical expression which contains some nonlinear terms like delays, interfaces etc., using symbolic math toolbox. An example of my code is as follows.
syms I1 I2 I3 I4 I5
terms = [I1.^2 I2-4 I1*I3-4 I4-8 I5]
theta = [1 2 1 1 5]'
Y = terms*theta
Here the term ''I3-4'' indicates the input I3 with a time delay of 4 seconds(I3(t-4)).I'm having the following issue
1. When I multiply terms and theta, it is giving me output as
Y = Y = I1^2+2*(I2-4)+I1*(I3-4)+(I4-8)+5*I5
I understood that matlab is using BODMAS rule, but is there any correct way to create delay terms using Symbolic Math Toolbox. Can some one help me regarding this issue?
Thanks

R2018a

### Raghunandan V (view profile)

on 4 Jun 2019

Hi,
There is mistake in the coe :)
And the term I1*I3-4 is not the same you explained. Here it follows BODMAS rule.
I1*I3-4 = (I1*I3) - 4
NOT I1*(I3-4)
Regards,
Raghunandan V

on 4 Jun 2019

matlab multiplied and simplified your codes;
Y = I1^2+2*(I2-4)+I1*(I3-4)+(I4-8)+5*I5
Y = I1^2+2*I2-8+I1*I3-4+I4-8+5*I5
simpliefied to
Y= I1^2+I3.I1+ ... %highest order
2.I2+ ... % I2 to I5
I4+ ...
5.I5+ ...
-20 %constant

Raghunandan V

### Raghunandan V (view profile)

on 4 Jun 2019
I1*(I3-4) is not I1*I3-4 but I1*I3- 4*I1. Check the calc
Raja Vardhan Reddy Kothakapu

### Raja Vardhan Reddy Kothakapu (view profile)

on 4 Jun 2019
yes, I understood that Matlab uses BODMAS rule. Some how I need to create this delay terms and pass them to the function handle , thats why I'm using I3-4 instead of I3(t-4). Maybe my question should be is there any correct way to create delay terms using symbolic math toolbax