Cho \(\cos \alpha = \frac{1}{4}.\) Giá trị của \(P = \frac{{\tan \alpha + 2\cot \alpha }}{{2\tan \alpha + 3\cot \alpha }}\) là:
A. \( – \frac{{17}}{{33}}.\)
B. \(\frac{{17}}{{33}}.\)
C. \(\frac{1}{2}.\)
D. \(\frac{{16}}{{33}}.\)
– Tính \({\tan ^2}\alpha \)
– Biến đổi \(P = \frac{{\tan \alpha + 2\cot \alpha }}{{2\tan \alpha + 3\cot \alpha }} = \frac{{{{\tan }^2}\alpha + 2}}{{2{{\tan }^2}\alpha + 3}}\)
Ta có: \(1 + {\tan ^2}\alpha = \frac{1}{{{{\cos }^2}\alpha }}\,\, \Rightarrow {\tan ^2}\alpha = \frac{1}{{{{\cos }^2}\alpha }} – 1 = 15.\)
Ta có: \(P = \frac{{\tan \alpha + 2\cot \alpha }}{{2\tan \alpha + 3\cot \alpha }} = \frac{{\frac{{\tan \alpha }}{{\cot \alpha }} + 2}}{{\frac{{2\tan \alpha }}{{\cot \alpha }} + 3}} = \frac{{{{\tan }^2}\alpha + 2}}{{2{{\tan }^2}\alpha + 3}} = \frac{{15 + 2}}{{2.15 + 3}} = \frac{{17}}{{33}}.\)
Chọn B.