The derivative of $$f(x, y)$$ at point $$(1, 2)$$ in the direction of vector $$\overrightarrow i + \overrightarrow j $$ is $$2\sqrt 2 $$ and in the direction of the vector $$ - 2\overrightarrow j $$ is $$-3.$$ Then the derivative of $$f(x,y)$$ in direction $$ - \overrightarrow i - 2\overrightarrow j $$ is

The function $$f\left( x \right) = {x^3} - 6{x^2} + 9x + 25$$ has

The function $$f\left( x \right) = \left| {x + 1} \right|$$ on the interval $$\left[ { - 2,0} \right]$$ is __________.

The void ratio of a soil sample can exceed unity.