The error message you are seeing indicates that the second input argument to the "diff" function should be a positive integer that specifies the order of the difference. In your code, you have passed the variable "y" as the second argument, which is causing the error.
The "diff" function calculates the differences between adjacent elements in an array, but it requires a scalar integer as the second input argument to specify the order of the difference. If you want to calculate the first-order difference of the "cf2a" array with respect to the "y" array, you can simply use the "diff" function without specifying the second argument, like this:
df2a = diff(cf2a);
This will calculate the first-order difference of the "cf2a" array, which should have one less element than the "cf2a" array.
However, if you want to calculate the derivative of the "cf2a" array with respect to the "y" array, you can use the "gradient" function instead of the "diff" function. Here's how you can modify your code to calculate the derivative of "cf2a":
L = 1; % assume the value of L to be 1
y = 100:400;
h2a = 310 + 7.85*y + 0.00194*y.^2;
cf2a = 1.5*h2a;
df2a = gradient(cf2a, y);
eq2 = df2a - L == 0;
This should calculate the derivative of "cf2a" with respect to "y" and store it in the "df2a" variable.
