\(C=\frac{1}{11\cdot16}+\frac{1}{16\cdot21}+...+\frac{1}{61\cdot66}=\frac{5}{5}\cdot\left(\frac{1}{11\cdot16}+\frac{1}{16\cdot21}+...+\frac{1}{61\cdot66}\right)\)

\(=\frac{1}{5}\cdot\left(\frac{5}{11\cdot16}+\frac{5}{16\cdot21}+...+\frac{5}{61\cdot66}\right)=\frac{1}{5}\cdot\left(\frac{1}{11}-\frac{1}{16}+\frac{1}{16}-\frac{1}{21}+...+\frac{1}{61}-\frac{1}{66}\right)\)

\(=\frac{1}{5}\cdot\left[\left(\frac{1}{11}-\frac{1}{66}\right)+\left(\frac{1}{16}-\frac{1}{16}\right)+...+\left(\frac{1}{61}-\frac{1}{61}\right)\right]\)

\(=\frac{1}{5}\cdot\left[\left(\frac{6}{66}-\frac{1}{66}\right)+0+...+0\right]=\frac{1}{5}\cdot\frac{5}{66}=\frac{1\cdot5}{5\cdot66}=\frac{1\cdot1}{1\cdot66}=\frac{1}{66}\)

Vậy \(C=\frac{1}{66}\)

Chúc bạn học tốt!^_^

Đặt \(t=x^2+x+2\) 

Suy ra \(\left(x^2+x\right)\left(x^2+x+4\right)-60=\left(t-2\right)\left(t+2\right)-60=t^2-8^2=\left(t-8\right)\left(t+8\right)\)

\(=\left(x^2+x-6\right)\left(x^2+x+10\right)=\left(x+3\right)\left(x-2\right)\left(x^2+x+10\right)\)

I/ Chia động từ : 

1. If he (be) lazy, he won't pass the exam

=> If he is lazy, he won't pass the exam.

2. Have your homework (do) yet?

=> Have your homework been done yet?

II/ Viết lại câu : 

1. Let's phone them.

=> How about phoning them.

2. The weather is awful.

=> I wish the weather weren't awful.

3. She can't sing English songs.

=> She wishes she could sing English songs.

4. My mother didn't stay at home last night.

=> I wish my mother had stayed at home last night.

5. What a pity! I am poor.

=> I wish I weren't poor.

là số 585 nha bạn 

Vì theo đề bài ta suy được chữ số đầu và chữ số cuối giống nhau và khác 0 nên bằng 5 mà chỉ có số 585 (thử lần lượt) thỏa mãn nên số cần tìm sẽ là 585

Số gạo nếp có là:

1500 x 1/6 = 250 (kg)

Số gạo tẻ có là: 

1500 - 250 = 1250 (kg)

Số túi gạo nếp có là:

250 : 2 = 125 (túi)

Số túi gạo tẻ có là:

1250 : 5 = 250 (túi)

Đáp số: 250 túi gạo tẻ; 125 túi gạo nếp

cho đúng nha

Xét với mọi n > 2 , ta có \(\frac{n}{n+2}< \frac{n-1}{n}\) (vì \(n^2< n^2+n-2\))

Áp dụng : \(A=\frac{1}{3}.\frac{4}{6}.\frac{7}{9}.\frac{10}{12}...\frac{208}{210}< \frac{1}{3}.\frac{3}{4}.\frac{6}{7}.\frac{9}{10}...\frac{207}{208}\)

Suy ra : \(A^2< \frac{1.4.7.10...208}{3.6.9.12...210}.\frac{1.3.6.9...207}{3.4.7.10...208}=\frac{1}{210}.\frac{1}{3}=\frac{1}{630}< \frac{1}{625}=\left(\frac{1}{25}\right)^2\)

Do đó \(A< \frac{1}{25}\)

những người sau, khi nào có đáp án khác thì hãy trả lời, nếu giống thì tôi cho sai, xem như trường hợp copy

1. If I see him , I will give him a lift

2. If Andy studies hard, he will pass his exam

3. If he eats all that, he will be ill

4. If I find your passport, I will telephone you at one

5. If he washes my car, I will give him $10

6. If I see John, I will tell him your message

7. If he reads in bad light, he will ruin his eyes.

8. If she needs some money, she can borrow mine

9. If he is late, we will go without him

10. If I have free time tomorrow , I will go shopping

11. If I had a lot of money, I would travel around the world

12. If you had arrived ten minutes earlier, you would have gotten a seat

13. If you had been here last week, you would have met my parents

14. If I had a map, I would be all right.

15. If she had known you were coming, she would have baked a cake

1 mol nguyên tử nguyên tố hóa học có \(6,023.10^{23}\) nguyên tử.

Ta có : \(n_{Al}=\frac{5,4}{27}=0,2\left(mol\right)\)

=> Số nguyên tử nhôm : \(0,2\times6,023.10^{23}=1,2046.10^{23}\)