:v bạn tách nhỏ đề ra và chụp ngang ra nhé

1 There are 5000 living languages in the world

2 It is Chinese

3 It is English

4 Yes, because I think English is interesting

V

1 He used to play the guitar at night

2 She wishes she had a pen pal

3 We started learning E 4 years ago

4 How long have you had that car

c)\(\left\{{}\begin{matrix}u_1+u_3=3\\u_1^2+u_3^2=5\end{matrix}\right.\)\(\Leftrightarrow\left\{{}\begin{matrix}u_1+u_3=3\\\left(u_1+u_3\right)^2-2u_1u_3=5\end{matrix}\right.\)\(\Leftrightarrow\left\{{}\begin{matrix}u_1+u_3=3\\u_1u_3=2\end{matrix}\right.\)\(\Leftrightarrow\left[{}\begin{matrix}\left\{{}\begin{matrix}u_1=2\\u_3=1\end{matrix}\right.\\\left\{{}\begin{matrix}u_1=1\\u_3=2\end{matrix}\right.\end{matrix}\right.\)

Làm nốt (sử dụng công thức: \(u_n=u_1+\left(n-1\right)d\) để tìm được công sai

\(S_n=nu_1+\dfrac{n\left(n-1\right)}{2}d\) để tính tổng 15 số hạng đầu)

d)\(\left\{{}\begin{matrix}u_1+u_2+u_3=14\\u_1u_2u_3=64\end{matrix}\right.\)\(\Leftrightarrow\left\{{}\begin{matrix}u_2-d+u_2+u_2+d=14\\\left(u_2-d\right)u_2\left(u_2+d\right)=64\end{matrix}\right.\)

\(\Leftrightarrow\left\{{}\begin{matrix}u_2=\dfrac{14}{3}\\\left(u_2^2-d^2\right)u_2=64\end{matrix}\right.\)\(\Leftrightarrow\left[{}\begin{matrix}\left\{{}\begin{matrix}\dfrac{14}{3}=u_2=u_1+d\\d=\dfrac{2\sqrt{889}}{21}\end{matrix}\right.\\\left\{{}\begin{matrix}\dfrac{14}{3}=u_1+d\\d=\dfrac{-2\sqrt{889}}{21}\end{matrix}\right.\end{matrix}\right.\) 

(Làm nốt,số xấu quá)

e)\(\left\{{}\begin{matrix}u_1+u_2+u_3=7\\u_1^2+u_2^2+u_3^2=21\end{matrix}\right.\)

\(\Leftrightarrow\left\{{}\begin{matrix}u_1+u_2+u_3=7\\u_1u_2u_3=\dfrac{21-\left(u_1+u_2+u_3\right)^2}{2}=-14\end{matrix}\right.\)

Làm như ý d)

Đề bài là: Tính cos2x 

Cảm ơn mn nhiều ạ!

`sin3x sinx+sin(x-π/3) cos (x-π/6)=0`

`<=> 1/2 (cos2x - cos4x) + 1/2(-sin π/6 + sin (2x-π/2)=0`

`<=> cos2x-cos4x-1/2+ sin(2x-π/2)=0`

`<=>cos2x-cos4x-1/2+ sin2x .cos π/2 - cos2x. sinπ/2=0`

`<=> cos2x - cos4x - cos2x = 1/2`

`<=> cos4x = cos(2π)/3`

`<=>` \(\left[{}\begin{matrix}4x=\dfrac{2\text{π}}{3}+k2\text{π}\\4x=\dfrac{-2\text{π}}{3}+k2\text{π}\end{matrix}\right.\)

`<=>` \(\left[{}\begin{matrix}x=\dfrac{\text{π}}{6}+k\dfrac{\text{π}}{2}\\x=-\dfrac{\text{π}}{6}+k\dfrac{\text{π}}{2}\end{matrix}\right.\)

\(sin3x.sinx+sin\left(x-\dfrac{\pi}{3}\right)cos\left(x-\dfrac{\pi}{6}\right)=0\)

\(\Leftrightarrow\dfrac{1}{2}cos2x-\dfrac{1}{2}cos4x+\dfrac{1}{2}sin\left(2x-\dfrac{\pi}{2}\right)+\dfrac{1}{2}sin\left(-\dfrac{\pi}{6}\right)=0\)

\(\Leftrightarrow\dfrac{1}{2}cos2x-\dfrac{1}{2}cos4x-\dfrac{1}{2}cos2x-\dfrac{1}{4}=0\)

\(\Leftrightarrow cos4x+\dfrac{1}{2}=0\)

\(\Leftrightarrow2cos^22x-1+\dfrac{1}{2}=0\)

\(\Leftrightarrow cos^22x=\dfrac{1}{4}\)

\(\Rightarrow cos2x=\pm\dfrac{1}{2}\)

bạn tự vẽ hình giúp mik nha

a. xét \(\Delta ADN\) và \(\Delta BAM\) có

AB=AD(gt)

\(\widehat{ADN}=\widehat{BAM}=90^o\)

DN=MA(N,M là trung điểm của cạnh DC,AD)

\(\Rightarrow\Delta ADN\sim\Delta BAM\left(c.g.c\right)\)

\(\Rightarrow\widehat{DNA}=\widehat{AMB}\)

mà:\(\widehat{DNA}+\widehat{DAN}=90^o\Rightarrow\widehat{BMA}+\widehat{DAN}=90^o\)

\(\Rightarrow\Delta MAI\) vuông tại I

\(\Rightarrow AI\perp MI\) hay \(MB\perp AN\)

b.ta có M là trung điểm của AD\(\Rightarrow AM=\dfrac{1}{2}AD=\sqrt{5}\)

trong \(\Delta MAB\) vuông tại A có

\(MB=\sqrt{AM^2+AB^2}=\sqrt{\sqrt{5^2}+\left(2\sqrt{5}\right)^2}=5\)

\(AM^2=MB.MI\Rightarrow MI=\dfrac{AM^2}{MB}=\dfrac{\sqrt{5^2}}{5^5}=0,2\)

\(AI.MB=AM.AB\Rightarrow AI=\dfrac{AM.AB}{MB}=\dfrac{\sqrt{5}.2\sqrt{5}}{5}\)=2

c.IB=MB-MI=5-0,2=4,8

\(S_{\Delta AIB}=\dfrac{AI.IB}{2}=\)\(\dfrac{2.4,8}{2}=4,8\)

\(S_{\Delta ADN}=\dfrac{AD.DN}{2}=\dfrac{2\sqrt{5}.\sqrt{5}}{2}=5\)

\(S_{\Delta ABCD}=\left(2\sqrt{5}\right)^2=20\)

\(S_{BINC}=S_{ABCD}-S_{\Delta AIB}-S_{\Delta DAN}\)=20-4,8-5=10,2

1 B

2 A

3 D

4 C

5 D

6 A

7 C

8 A

9 B

10 A

Bài 3:

a. \(R=R1+R2=15+30=45\Omega\)

b. \(\left\{{}\begin{matrix}I=U:R=9:45=0,2A\\I=I1=I2=0,2A\left(R1ntR2\right)\end{matrix}\right.\)

c. \(\left\{{}\begin{matrix}U1=R1.I1=15.0,2=3V\\U2=R2.I2=30.0,2=6V\end{matrix}\right.\)

Bài 4:

\(I1=U1:R1=6:3=2A\)

\(\Rightarrow I=I1=I2=2A\left(R1ntR2\right)\)

\(U=R.I=\left(3+15\right).2=36V\)

\(U2=R2.I2=15.2=30V\)