How to find following integral answer in matlab

1 次查看（过去 30 天）

显示更早的评论

Alireza Babaei 2020-3-30

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/514159-how-to-find-following-integral-answer-in-matlab

评论： David Goodmanson 2020-3-31

在 MATLAB Online 中打开

Dear scholars,

I need to find soln of integral:

int((f1(x))^3 * f2(x), x, 0, L)

but matlab can not solve it,

Any ideas?

I also attach the code here

(fi(x) are all combintaions of trigonometrix and hyperbolic functions, and due to orthogonality we know that int(f1(x) * f2(x), x, 0, L)) = 0, but I need to make sure whenever the first function is cubed)

clc
clear all
close all
format long 
 L = 100e-3;
 Areap = 2*(b*hp);
Areas = b*hs;
Areat = Areas + Areap;
roAt = ros*Areas + rop*Areap;
landaL = [1.875104, 4.694091, 7.854757];
landa = landaL./L;
syms x
    a1 = (cosh(landaL(1)+cos(landaL(1))));
    a2 = (sinh(landaL(1))+sin(landaL(1)));
    % phi = (1/sqrt(roAt*L)) * (cosh(landa*x)-cos(landa*x)-(a1/a2)*(sinh(landa*x)-sin(landa*x)));
 phi(1) = (1/sqrt(roAt*L)) * (cosh(landa(1)*x)-cos(landa(1)*x)-(a1/a2)*(sinh(landa(1)*x)-sin(landa(1)*x)));
b1 = (cosh(landaL(2)+cos(landaL(2))));
    b2 = (sinh(landaL(2))+sin(landaL(2)));
    % phi = (1/sqrt(roAt*L)) * (cosh(landa*x)-cos(landa*x)-(a1/a2)*(sinh(landa*x)-sin(landa*x)));
 phi(2) = (1/sqrt(roAt*L)) * (cosh(landa(2)*x)-cos(landa(2)*x)-(b1/b2)*(sinh(landa(2)*x)-sin(landa(2)*x)));
% A01 = round(int(roAt*phi(1)*phi(1),x,0,L))
 A11 = round(int(roAt*phi(1)*phi(2),x,0,L))
 % A11 is = 0 showing and proving the orthogonality trait ...
  A21 = int(roAt*(phi(1)^2)*phi(2),x,0,L)
  %A21 = round(int(roAt*(phi(1)^2)*phi(2)),x,0,L)
 

1 个评论
显示 -1更早的评论隐藏 -1更早的评论

David Goodmanson 2020-3-31

在 MATLAB Online 中打开

Hi Alireza,

you did not supply the constants needed to calculate roAt, but since that quantity is only used as a normalization constant it does not affect whether an integral is zero or not. I just used roAt = 1. Now

A11 = double(int(roAt*phi(1)*phi(2),x,0,L))
A11 =
    0.3165

so it's nonzero. Numerical integration gives the same result. That is more or less expected since the trig functions do not have an integral number of oscillations in the span from 0 to L. How do you justify using round in this situation?

请先登录，再进行评论。

请先登录，再回答此问题。