hypot - Euclidean distance
double hypot(double x, double y)
Hypot(x, y) returns sqrt(x*x+y*y) computed in such a way that underflow
will not happen, and overflow occurs only if the final result deserves
hypot(Inf, v) = hypot(v, Inf) = +Inf for all v, including NaN.
ERROR (due to Roundoff, etc.)
Below 1 ulps. (ulp = Units in the Last Place).
hypot(x, y) = Inf if x or y is +Inf or -Inf.
hypot(x, y) = NaN if x or y is NaN (but not +Inf).