Using abs in a polynomious

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Answer 1

[Walter Roberson]

You cannot pass a function handle to polyval.

"The input argument p is a vector of length n+1 whose elements are the coefficients in descending powers of the polynomial to be evaluated."

Notice that your ap already does a polyval, so even if you invoked ap somehow it would be a mistake to use it in polyval afterwards.

You should just be using

y2 = abs(polyval(x2,p));
trapz(x2, y2)