Which of the following is an essential amino acid?

For a suitably chosen real constant $a$,let a function $f: R-\{-a\} \rightarrow R$ be defined by $f(x) = \frac{a-x}{a+x}$. Further,suppose that for any real number $x \neq -a$ and $f(x) \neq -a$,$(f \circ f)(x) = x$. Then $f(-\frac{1}{2})$ is equal to

In the circuit shown in the figure below, which of the following statements is incorrect?

Fresnel diffraction is produced due to light rays falling on a small obstacle. The intensity of light at a point on a screen beyond an obstacle depends on

Which of the following statements are true about acceleration due to gravity?
$(a)$ $g$ decreases in moving away from the centre if $r > R$
$(b)$ $g$ decreases in moving away from the centre if $r < R$
$(c)$ $g$ is zero at the centre of the Earth
$(d)$ $g$ decreases if the Earth stops rotating on its axis

What are the products formed when phenyl ethyl ether is heated with concentrated $HBr$?