JEE Math Practice Question

Let f(x) = $${x \over {\sqrt {{a^2} + {x^2}} }} - {{d - x} \over {\sqrt {{b^2} + {{\left( {d - x} \right)}^2}} }},\,\,$$ x $$\, \in $$ R, where a, b and d are non-zero real constants. Then :

1. A.f is an increasing function of x
2. B.f is neither increasing nor decreasing function of x
3. C.f ' is not a continuous function of x
4. D.f is a decreasing function of x