Tính lim x → π 4 sin x − cos x tan ( x − π 4 )

Ta có:

\[\sin {\rm{x}} - \cos {\rm{x}} = \sqrt 2 \sin \left( {{\rm{x}} - \frac{{\rm{\pi }}}{4}} \right)\]

\[\tan \left( {{\rm{x}} - \frac{{\rm{\pi }}}{4}} \right) = \frac{{\sin \left( {{\rm{x}} - \frac{{\rm{\pi }}}{{\rm{4}}}} \right)}}{{\cos \left( {{\rm{x}} - \frac{{\rm{\pi }}}{{\rm{4}}}} \right)}}\]

\[\mathop {\lim }\limits_{{\rm{x}} \to \frac{{\rm{\pi }}}{4}} \frac{{\sin {\rm{x}} - \cos {\rm{x}}}}{{\tan \left( {{\rm{x}} - \frac{{\rm{\pi }}}{4}} \right)}} = \mathop {\lim }\limits_{{\rm{x}} \to \frac{{\rm{\pi }}}{4}} \frac{{\sqrt 2 \sin \left( {{\rm{x}} - \frac{{\rm{\pi }}}{4}} \right).\cos \left( {{\rm{x}} - \frac{{\rm{\pi }}}{4}} \right)}}{{\sin \left( {{\rm{x}} - \frac{{\rm{\pi }}}{4}} \right)}} = \mathop {\lim }\limits_{{\rm{x}} \to \frac{{\rm{\pi }}}{4}} \sqrt 2 \cos \left( {{\rm{x}} - \frac{{\rm{\pi }}}{4}} \right) = \sqrt 2 \]

Chọn đáp án B

Đáp án cần chọn là: B