built-in functions, abs, atan, ceiling, cos, exp, float, floor, log, round, sign, sin, sqrt, tan, truncate
A small number of mathematical functions are built into Prolog, including:

FunctionMeaning
abs(Exp)absolute value of Exp : i.e. Exp if Exp ≥ 0, –Exp if Exp < 0
atan(Exp)arctangent (inverse tangent) of Exp : result is in radians
cos(Exp)cosine of the Exp : Exp is in radians
exp(Exp)eExp : e is 2.71828182845…
log(Exp)natural logarithm of Exp : i.e. logarithm to the base* e
sin(Exp)sine of the Exp : Exp is in radians
sqrt(Exp)square root of the Exp
tan(Exp)tangent of the Exp: Exp is in radians
sign(Exp)sign (+1 or –1) of the Exp: sign(–3) = –1 = sign(–3.7)
float(Exp)float of the Exp: float(22) = 22.0 - see also float the predicate
floor(Exp)largest integer ≤ Exp: floor(1.66) = 1
truncate(Exp)remove fractional part of Exp: truncate(–1.5) = –1, truncate(1.5) = 1
round(Exp)round Exp to nearest integer: round(1.6) = 2, round(1.3) = 1
ceiling(Exp)smallest integer ≥ Exp: ceiling(1.3) = 2

These functions should be used in a context where they will actually be evaluated, such as following is or as part of an arithmetic comparison operator like =:= or >.

Example:

?- X is sqrt(2).
X = 1.41421 
Compare this with the following, where sqrt(2) is not evaluated, because = does not evaluate its arguments.
?- X = sqrt(2).
X = sqrt(2) 

Another example:
?- X is log(3+2).
X = 1.60944.

These mathematical functions may correspond to arity 2 built-in predicates: for example, one can do this:

?- sqrt(2, X).
X = 1.41421
Some versions of SWI Prolog (e.g. 5.6.47) implement many of these arity 2 predicates, but not e.g. exp/2.

* High School Maths Reminder Service: if you want the logarithm to base a, divide log(Exp) by log(a). E.g. log10(X) = log(X)/log(10), and log2(X) = log(X)/log(2).