preval takes three arguments : an expression F
the variable x, and two expressions a and b.
preval computes F|x=b−F|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.