Solving an equation for a variable

2021 2 22
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更新时间：2021 2 23
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I am given the following equation,
fx =@(x) ((15*3)/(3*pi^4*70*5.29E-6))*(48*3^3*cos((pi*x)/(2*3))-(48*3^3)+(3*pi^3*3*x^2)-(pi^3*x^3));
If I wanted to find the value of x at say fx = 10, is there a shortcut to do so in MATLAB?

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Walter Roberson
Walter Roberson 2021-2-22

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The trick is to create a new function, such as fx10, that is fx(x)-10 and then do root finding (fzero, fsolve, vpasolve) on that new function.

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Walter Roberson
Walter Roberson 2021-2-22
Note: there are two roots that are close to 0, and another root closer to 8.
Raymond Elliott
Raymond Elliott 2021-2-22
I am trying to use the root finding methods you provided, but it is asking for at least two inputs. How do you know what to put for the second input?
Raymond Elliott
Raymond Elliott 2021-2-22
Oh nevermind I see now that the second input is where you put the value of the function you are trying to solve for. Thank you for your guidance!
Raymond Elliott
Raymond Elliott 2021-2-22
One last question, how were you able to find that there were two roots that were close to zero? Did you plot the function to see or did you do it some other way?
Walter Roberson
Walter Roberson 2021-2-23
The second input is a starting guess.
I used a negative starting guess and a modest positive starting guess. Afterwards I plotted to confirm.

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