a: Xét ΔBAC có 

D là trung điểm của AB

M là trung điểm của AC

Do đó: DM là đường trung bình của ΔABC

Suy ra: DM//BC và \(DM=\dfrac{BC}{2}=3.5\left(cm\right)\)

a: \(A=4-\sin^245^0+2\cdot cos^260^0-3\cdot\cot^345^0\)

\(=4-\left(\frac{\sqrt2}{2}\right)^2+2\cdot\left(\frac12\right)^2-3\cdot1^3\)

\(=4-\frac12+2\cdot\frac14-3=4-3=1\)

b: \(B=tan45^0\cdot cos30^0\cdot\cot30^0\)

\(=1\cdot\frac{\sqrt3}{2}\cdot\sqrt3=\frac32\)

c: \(C=cos^215^0+cos^225^0+\cdots+cos^275^0\)

\(=\left(cos^215^0+cos^275^0\right)+\left(cos^225^0+cos^265^0\right)+\left(cos^235^0+cos^255^0\right)+cos^245^0\)

\(=1+1+1+\left(\frac{\sqrt2}{2}\right)^2=3+\frac12=\frac72\)

d: \(D=\sin^210^0+\sin^220^0+\cdots+\sin^280^0\)

\(=\left(\sin^210^0+\sin^280^0\right)+\left(\sin^220^0+\sin^270^0\right)+\left(\sin^230^0+\sin^260^0\right)+\left(\sin^240^0+\sin^250^0\right)\)

=1+1+1+1

=4

đọc đề mà quạo lun ă:>

dễ mà tự lm ik:333

1,C

2,C

3,C

4,A

1,We have learnt E for 4 years

2,It has been 3 years since I last saw her

3,He has not smoked since last year

4,Loan last wrote to you a month ago

5,Mai began singing when she was 7

6,They stopped produced this...toy long time ago

7, It is 2 hours since we watched TV

8,The last time I heard him was August

9,The last time Mr Brown taught E at this school was on January 10^th

1, That
2, This
3, that
4, those
5, these - that
6, these
7, this
8, that
9, that
10, this
11, those
12, this
13, it
14, these
15, them

16, those

this/that dùng cho ng, vật số ít

this: dùng cho ng, vật ở gần

that: dùng cho ng, vật ở xa

these/those dùng cho ng, vật số nhiều

these: ở gần

those: ở xa

a: ĐKXĐ: \(\begin{cases}x\ge0\\ x<>4\end{cases}\)

\(P=\left(\frac{\sqrt{x}+3}{\sqrt{x}-2}+\frac{\sqrt{x}-1}{\sqrt{x}+2}+\frac{4\sqrt{x}-4}{4-x}\right):\left(1+\frac{5}{\sqrt{x}-2}\right)\)

\(=\frac{\left(\sqrt{x}+3\right)\left(\sqrt{x}+2\right)+\left(\sqrt{x}-1\right)\left(\sqrt{x}-2\right)-4\sqrt{x}+4}{\left(\sqrt{x}-2\right)\left(\sqrt{x}+2\right)}:\frac{\sqrt{x}-2+5}{\sqrt{x}-2}\)

\(=\frac{x+5\sqrt{x}+6+x-3\sqrt{x}+2-4\sqrt{x}+4}{\left(\sqrt{x}-2\right)\left(\sqrt{x}+2\right)}:\frac{\sqrt{x}+3}{\sqrt{x}-2}\)

\(=\frac{2x-2\sqrt{x}+12}{\left(\sqrt{x}+2\right)\left(\sqrt{x}+3\right)}\)

b: \(P>\frac12\)

=>\(\frac{2x-2\sqrt{x}+12}{x+5\sqrt{x}+6}-\frac12>0\)

=>\(\frac{4x-4\sqrt{x}+24-x-5\sqrt{x}-6}{2\left(x+5\sqrt{x}+6\right)}>0\)

=>\(3x-9\sqrt{x}+18>0\)

=>\(x-3\sqrt{x}+6>0\)

=>\(\left(\sqrt{x}-\frac32\right)^2+\frac{15}{4}>0\) (luôn đúng với mọi x thỏa mãn ĐKXĐ)

=>x>=0 và x<>4