Calculating derivative and integration using matlab

I am trying to find second derivative of a function, but while initializing my symbols i am getting the following error:

Error using ==> subsindex
Function 'subsindex' is not defined for valu开发者_StackOverflow中文版es of class 'sym'.

The commands I am using are:

syms x a b c L;
u = (a*x(x-L))+(b*x((x^2)-(L^2)))+(c*x((x^3)-(L^3)));

"u" is my function.

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I don't know much about MATLAB's symbolic capabilities, but that error is coming from the pieces like

x(x-L)

which MATLAB is interpreting as an indexing operation. Did you mean multiplication there? I.e.

x*(x-L)