### 5.12.18 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=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.

Input :

preval(x`^`

2+x,2,3)

Output :

6