\(\left|cosx\right|=\dfrac{1}{2}\)

\(\Leftrightarrow\left[{}\begin{matrix}cosx=\dfrac{1}{2}\\cosx=-\dfrac{1}{2}\end{matrix}\right.\)

\(\Leftrightarrow\left[{}\begin{matrix}x=\pm\dfrac{\pi}{3}+k2\pi\\x=\pm\dfrac{\pi}{2}+k2\pi\end{matrix}\right.\)

\(\Leftrightarrow x=\pm\dfrac{\pi}{3}+k2\pi\)

\(\left|cosx\right|=\dfrac{1}{2}\)

\(\Leftrightarrow cos^2x=\dfrac{1}{4}\)

\(\Leftrightarrow\dfrac{1+cos2x}{2}=\dfrac{1}{4}\)

\(\Leftrightarrow cos2x=-\dfrac{1}{2}\)

\(\Rightarrow2x=\pm\dfrac{2\pi}{3}+k2\pi\)

\(\Rightarrow x=\pm\dfrac{\pi}{3}+k\pi\)

ok nha

A = 1/2 + 1/6 + 1/16 + ... + 1/4084441   có : 2021 - 1 + 1 = 2021 số

1 = 1/2021 + 1/2021 + ... + 1/2021   có 2021 số 

vậy 1 > A

a)

Kim loại : $Pb$

Phi kim : $Cl_2,C$

b)

Axit : $CH_3COOH$

Oxit : $Fe_2O_3,CO,N_2O_5$

Bazo : $Ni(OH)_2$

Muối : $KMnO_4,MnSO_4,Na_2HPO_4,NH_4Cl$

Tham khảo

1. Someone will clean the room today. =>THE ROOM WILL BE CLEANED TODAY

2. Someone might steal the car. =>THE CAR MIGHT BE STOLEN

3. They had to cut down that tree.=>THAT TREE HAD TO BE CUT DOWN

4. They're going to demolish the old houses.=>THE OLD HOUSES ARE GOING TO BE DEMOLISHED

5. We can't restore the picture. =>THE PICTURE CAN'T BE RESTORED

6. You must make an appointment in advance.=>AN APPOINTMENT MUST BE MADE IN ADVANCE

7. I don't want people to make me a fool. =>I DON'T WANT TO BE MADE A FOOL

8. Someone has to look after the garden.=>THE GARDEN HAS TO BE LOOKED AFTER

9. He wants everybody to serve him. =>HE WANTS TO BE SERVED

10. They're going to interview him next week=>HE IS GOING TO BE INTERVIEWED NEXT WEEK

Xét \(1-A=1-\frac{2x+1}{x^2+2}=\frac{x^2-2x+1}{x^2+2}=\frac{(x-1)^2}{x^2+2}\geq 0 (\ do\ (x-1)^2\geq 0, x^2+2>0)\)

Suy ra \(A\leq 1\)

Dấu = xảy ra khi x=1

Thnks

  đây nhé làm 3 bài đầu 

5:

B=(x+y)^2-2xy

=225-2*(-100)=425

6:

a: C=(39+61)^2=100^2=10000

b: D=50^2-50^2+1=1