Trying to solve for x

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Christina Kersten 2019-4-13

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关闭： MATLAB Answer Bot 2021-8-20

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So basically I have this equation, and I'm trying to find where the second derivative of it equals 0, but I keep getting that x = 0 and I know it's not. What am I doing wrong?

syms x % Defining unknown variable to solve for
eqn = 1/(7.221*10^(-9)*x^3 - 2.353E-5*x^2 - 2.094E-3*x + 132.747); % Equation taken from polyfit
ddc = diff(diff(eqn)) == 0;
sol = solve(ddc,x);
compliance = double(sol);