1,999699365

1+1=2 và mk đã cs ny rùi

Trả lời :

1 + 1

= 2

#Hok tốt

\(A=\frac{cos7x-cos8x-cos9x+cos10x}{sin7x-sin8x-sin9x+sin10x}=\frac{(cos10x+cos7x)-\left(cos9x+cos8x\right)}{\left(sin10x+sin7x\right)-\left(sin9x+sin8x\right)}.\) 

     \(=\frac{2cos\frac{17x}{2}cos\frac{3x}{2}-2cos\frac{17x}{2}cos\frac{x}{2}}{2sin\frac{17x}{2}cos\frac{3x}{2}-2sin\frac{17x}{2}cos\frac{x}{2}}=\frac{2cos\frac{17x}{2}\left(cos\frac{3x}{2}-cos\frac{x}{2}\right)}{2sin\frac{17x}{2}\left(cos\frac{3x}{2}-cos\frac{x}{2}\right)}=cotan\frac{17x}{2}.\)  

tôi biết bạn nhưng bạn không biết

-phê lòi

a : b +C _4= b + a

Em có cách này,anh check lại nhé!

Theo nguyên lí Dirichlet,trong ba số (a - 1) ; (b - 1); (c - 1) tồn tại hai số có tích không âm.

Không mất tính tổng quát,giả sử \(\left(a-1\right)\left(b-1\right)\ge0\Rightarrow ab\ge a+b-1\)

\(\Rightarrow abc\ge ac+bc-c\)

Suy ra \(VT\ge a^2+b^2+\left(c^2+ac+bc-c\right)\)

\(=a^2+b^2+c\left(a+b+c-1\right)=a^2+b^2+2c\)

Ta cần chứng minh \(a^2+b^2+2c\ge4\)

Thật vậy,BĐT \(\Leftrightarrow\left(a^2+1\right)+\left(b^2+1\right)+2c\ge6\)

Áp dụng BĐT Cô si (AM-GM) ta được: \(VT\ge2\left(a+b+c\right)=2.3=6\)(Q.E.D)

1 the aim of the culture festival is ....... friendship between the two countries 

A . promote       B . promoting             C . to promote               D. being promoted

2. people he ......... turned out to be only fair - weather friends 

A. trusted              B. has trusted             C was trusting               D had trusted 

3. How can you let such a silly incident ............ your friendship 

A. wreck             B to wreck                C wrecking               D that wrecks  

4. it has  become necessary ....... water in the metropolitan area because of the severe drought 

A. rationing              B ration                   C to ration              D to have rationed 

5 All the pasengers were made.............. their seat belts during the turbulence 

A. buckle                     B to buckle                  C buckling                   D for buckling 

6. .......... good ice cream , you need to use a lot of cream 

A Make                  B making                 C to make                        D for make 

7 , i got my friend ........... her car for the weekend 

A . to let me to borrow            B to let me borrow              C let me borrow                 D let me to borrow 

8 they .......... good friends , but they have fallen out recently

A . used to be                  B would be                 C were              D are

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