Let $x = at^2 - 1$, where $a > 0$ and $y = t^3 + 1$. If at $t = 1$, $\dfrac{d^2y}{dx^2} = \dfrac{3}{16}$, then the value of $a$ is...

If $y = \cot^{-1}\left(\dfrac{1 + \sin 5x}{\cos 5x}\right)$, then the value of $\dfrac{dy}{dx}$ is

The derivative of $\log_{10} x$ with respect to $\log_x 10$ is

A tank with a rectangular base and rectangular sides, open at the top is made. Depth of the tank is $4$ m and its volume is $36$ cubic meters. For making a tank cost of base material used is Rs. $100$ per sq. meter and that of sides is Rs. $50$ per sq. meter. Then minimum cost of tank is ..............