mathematical expression from string input

Question

Md. Ashikur Rahman Any 2021-4-17

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/804656-mathematical-expression-from-string-input

评论： Stephan 2021-4-19

I want to make a equation solver which will take any type of mathmatical equation as input .and return the roots.For that reason ,first the program need to read the equation.I want to input the equation in string format.then matlab will consider this as a function of x.

for example if i give input as

(sin(sqrt(x)+a) * e^sqrt(x)) / sqrt(x)

Matlab will read this as

and then i will plot the curve ,and the intersecting point with x axis is the solution of the equation.

How to do that?

Thanks in Advance

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Answer 1

Stephan 2021-4-17

1
链接

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https://ww2.mathworks.cn/matlabcentral/answers/804656-mathematical-expression-from-string-input#answer_677331

编辑：Stephan 2021-4-17

在 MATLAB Online 中打开

https://de.mathworks.com/help/symbolic/str2sym.html

>> fun = str2sym("(sin(sqrt(x)+a) * e^sqrt(x)) / sqrt(x)")
fun =
 
(e^(x^(1/2))*sin(a + x^(1/2)))/x^(1/2)

>> pretty(fun)
 sqrt(x)
e        sin(a + sqrt(x))
-------------------------
         sqrt(x)

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Md. Ashikur Rahman Any 2021-4-18

The equation has multiple solution.But it only shows one.

How to show multiple solutions as output.

I know there are infinite number of solution.But if i want to get solution in a sfecific range ,how to get that??

Stephan 2021-4-19

在 MATLAB Online 中打开

Make assumptions on x and solve then:

syms x a
fun = str2sym("(sin(sqrt(x)+a) * exp(sqrt(x))) / sqrt(x)")
fun = subs(fun, a, 1)
assume([x>25, x<30])
intersect = solve(fun==0)
fplot(fun,[25,30])
xline(double(intersect),'k--')
yline(0,'k--')