Question

 

Answer 1

lx

Answer 2

lx

Answer 3

12.

\(\pi< a< \dfrac{3\pi}{2}\Rightarrow sina< 0\)

\(cos\left(a+2021\pi\right)=cos\left(a+\pi+2020\pi\right)=cos\left(a+\pi\right)=-cosa\)

\(\Rightarrow cosa=-\dfrac{1}{3}\Rightarrow sina=-\sqrt{1-cos^2a}=-\dfrac{2\sqrt{2}}{3}\)

\(\Rightarrow cota=\dfrac{cosa}{sina}=\dfrac{1}{2\sqrt{2}}\)

13.

\(cos^22x=1-sin^22x\)

14.

\(cos6a=1-6sin^2a\) là công thức sai

15.

\(M=cos\left(115^0+365^0\right)=cos\left(480^0\right)\)

16.

\(sina.cosb=\dfrac{1}{2}\left[sin\left(a-b\right)-cos\left(a+b\right)\right]\) là công thức sai