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2.12.18  Evaluate a primitive at boundaries: preval

preval takes three arguments : an expression F depending on the variable x, and two expressions a and b.
preval computes F|x=bF|x=a.
preval is used to compute a definite integral when the primitive F of the integrand f is known. Assume for example that F:=int(f,x), then preval(F,a,b) is equivalent to int(f,x,a,b) but does not require to compute again F from f if you change the values of a or b.
Input :

preval(x^2+x,2,3)

Output :

6

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