Bài 3 trang 15 SGK Toán 11 tập 1 - Cánh diều

Đề bài

Tính các giá trị lượng giác (nếu có) có mỗi góc sau:

a)     \(\frac{\pi }{3} + k2\pi \,\,\left( {k \in Z} \right)\)

b)     \(\frac{\pi }{3}+\left( 2k+1 \right)\pi \,\,\left( k\in \mathbb{Z} \right)\)

c)     \(k\pi \,\,\left( {k \in Z} \right)\)

d)     \(\frac{\pi }{2} + k\pi \,\,(k \in Z)\)

Lời giải chi tiết

a)

\(\begin{array}{l}\cos \left( {\frac{\pi }{3} + k2\pi \,} \right) = \cos \left( {\frac{\pi }{3}} \right) = \frac{1}{2}\\\sin \left( {\frac{\pi }{3} + k2\pi \,} \right) = \sin \left( {\frac{\pi }{3}} \right) = \frac{{\sqrt 3 }}{2}\\\tan \left( {\frac{\pi }{3} + k2\pi \,} \right) = \frac{{\sin \left( {\frac{\pi }{3} + k2\pi \,\,} \right)}}{{\cos \left( {\frac{\pi }{3} + k2\pi \,\,} \right)}} = \sqrt 3 \\\cot \left( {\frac{\pi }{3} + k2\pi \,\,} \right) = \frac{1}{{\tan \left( {\frac{\pi }{3} + k2\pi \,\,} \right)}} = \frac{{\sqrt 3 }}{3}\end{array}\)

b) Các giá trị lượng giác của góc lượng giác \(\frac{\pi }{3}+\left( 2k+1 \right)\pi \,\,\left( k\in \mathbb{Z} \right)\)

$ \cos \left[\frac{\pi}{3}+(2 \mathrm{k}+1) \pi\right]=\cos \left(\frac{\pi}{3}+\pi+2 \mathrm{k} \pi\right)=\cos \left(\frac{\pi}{3}+\pi\right)=-\cos \frac{\pi}{3}=-\frac{1}{2}$

$\sin \left[\frac{\pi}{3}+(2 \mathrm{k}+1) \pi\right]=\sin \left(\frac{\pi}{3}+\pi+2 \mathrm{k} \pi\right)=\sin \left(\frac{\pi}{3}+\pi\right)=-\sin \frac{\pi}{3}=-\frac{\sqrt{3}}{2}$

$\tan \left[\frac{\pi}{3}+(2 \mathrm{k}+1) \pi\right]=\tan \frac{\pi}{3}=\sqrt{3}$;

$\tan \left[\frac{\pi}{3}+(2 \mathrm{k}+1) \pi\right]=\cot \frac{\pi}{3}=\frac{\sqrt{3}}{3}$

c)

\(\begin{array}{l}\cos \left( {k\pi \,} \right) = \left[ \begin{array}{l} - 1\,\,\,\,\,\,\,\,\,\,\,\,\,;k = 2n + 1\\1\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,;k = 2n\,\,\,\end{array} \right.\\\sin \left( {k\pi \,} \right) = 0\\\tan \left( {k\pi \,} \right) = \frac{{\sin \left( {k\pi \,\,} \right)}}{{\cos \left( {k\pi \,\,} \right)}} = 0\\\cot \left( {k\pi \,\,} \right)\end{array}\)

d)

\(\begin{array}{l}\cos \left( {\frac{\pi }{2} + k\pi \,} \right) = 0\\\sin \left( {\frac{\pi }{2} + k\pi \,} \right) = \left[ \begin{array}{l}\sin \left( { - \frac{\pi }{2}} \right)\, =  - 1\,\,\,\,\,\,\,;k = 2n + 1\\\sin \left( {\frac{\pi }{2}\,} \right)\, = 1\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,;k = 2n\,\,\,\end{array} \right.\\\tan \left( {\frac{\pi }{2} + k\pi \,} \right)\\\cot \left( {\frac{\pi }{2} + k\pi \,\,} \right) = 0\end{array}\)