### 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=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`