List of top Mathematics Questions on Differentiability asked in BCECE

Let $ f(x)=\left\{ \begin{matrix} \frac{\tan x-\cot x}{x-\frac{\pi }{4}}, & x\ne \frac{\pi }{4} \\ a, & x=\frac{\pi }{4} \\ \end{matrix} \right. $ the value of a so that $ f(x) $ is continuous at $ x=\frac{\pi }{4} $