A function f defined by $$f(x) = \ln \left( {\sqrt {{x^2} + 1} - x} \right)$$ is

The domain of the function f defined by

f(x) = log_x 10 is

For r > 0, f(r) is the ratio of perimeter to area of a circle of radius r. Then, f(1) + f(2) is equal to

If f(x) = 3^{1 + x}, then f(x) f(y) f(z) is equal to