Help!: int(expr,var) function returns the int(expr,var) function

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Daniel Møgelbjerg 2021-4-16

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https://ww2.mathworks.cn/matlabcentral/answers/803881-help-int-expr-var-function-returns-the-int-expr-var-function

评论： Daniel Møgelbjerg 2021-4-20

When i try to make a fucntion described by two indefinite integrals, one of the indefinite integrals returns without being solved.

The variable x an angle between 0 and pi/2.

F_1 is a force.

R_1 and R_2 are lenghts.

%% Variables
syms x;
F_1 = 200; %N
R_1 = 30; % mm
L = 40; % mm
H = 20; % mm
R_2 = 20*(4*cos(x)^2 + sin(x)^2)^(1/2);
%% Equations
m_1(x)= F_1*sin(x)*R_1;
md_1(x)=sin(x)*R_1;
m_2(x)=F_1*R_2*cos(x);
md_2(x)=R_2*cos(x);
ex_1=R_1*m_1(x)*md_1(x);
ex_2=simplify(R_2*m_2(x)*md_2(x))
%% Integration
f(x)=int(ex_1,x) + int(ex_2,x)

f(x) =

2700000*x - 1350000*sin(2*x) - int(1600000*(4 - 3*sin(x)^2)^(3/2)*(sin(x)^2 - 1), x) - It should solve this one, but indstead it just spit out int(expresion,var) just as it was inserted.

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Answer 1

Cris LaPierre 2021-4-17

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/803881-help-int-expr-var-function-returns-the-int-expr-var-function#answer_677256

That likely means it couldn't solve that one.

Consider using one of the techniques mentioned in these two examples.

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Daniel Møgelbjerg 2021-4-20

We did it a little different, after realizing that our R_2 in fact was not a variable depending on x, but just another constant value. We are sorry for the eventually wasted time, but thanks a lot for the help.

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