question_answer

\[\underset{x\to \pi /4}{\mathop{\lim }}\,\frac{\tan x-1}{\cos 2x}\] is equal to

A) 1                                             

B) 0

C)  -2                                          

D) -1

Correct Answer: D

Solution :

\[\Delta A=\left( \frac{dA}{dx} \right)\Delta x=\left( 12x \right)\Delta x\]              \[=12x\times \left( \frac{mx}{100} \right)=\frac{12{{x}^{2}}m}{100}\]                                            \[\therefore \] \[=\frac{2m}{100}\times 6{{x}^{2}}\] \[=\pm 4\sqrt{2}\] \[x=\pm \frac{8}{\sqrt{2}}\] = - 1