1.To: msha@gmail.com

Subject: Group homework submission - Group 1 - Class 8A

Attachment: Homework_Group1. docx

Dear Mrs.Ha,

My name is ......, the leader of group 1, class 8A. I would like to submit our group homework on the topic global warming.

Attached is the file of our homework. All information of our group's members like names and student codes is included in the file.

Please let us know if you have any more requirements.

Yours sincerely,

     Minh

2.I agree with idea advanced technology will help students greatly in studying by themselves at home. Firstly, studying at home will help us save so much money. We don’t have to buy so many books and school supplies. It will help us save time and energy too because we don’t have to spend much time on travelling to school. Secondly, you can see there is a lot of traffic on the way especially in rush hours. There are always accidents on the road. Studying at home will help us avoid dangers on the way to school and make traffic less heavy. In addition, we can search for information easily and quickly with a tablet. It’s very convenient. It helps us to save time and we can finish our projects quicker and make them more informative. In conclusion, I think advanced technology really helps us in studying and it brings us more joy in learning new knowledge.

3.I disagree with this statement.Modern technology is mostly seen as blessing for learners.It made learning easier .The internet can provide you with ìnformation in any field,you can easily find that online .Students who are using technology now will find it easy to complete in the future.But the availability of all types of information online ,many students are  becoming lazy.Some students even do not attend classes,they think that teachers are no longer nesessary 

Gọi số quần áo may theo kế hoạch là x(bộ,x>0)

       số quần áo may thực tế là x + 60 (bộ)

Thời gian dự định may xong là \(\dfrac{x}{90}\)(ngày)

Thời gian thực tế may xong là : \(\dfrac{x+60}{120}\)(ngày)

Theo đề bài ,ta có phương trình :\(\dfrac{x}{90}-\dfrac{x+60}{120}=9\)

giải ra ta được x = 3420(tm)

Vậy số quần áo may theo kế hoạch là 3420 bộ

\(\dfrac{x-y}{z}=\dfrac{3y}{x-z}=\dfrac{x}{y}=\dfrac{x-y+3y+x}{z+x-z+y}=\dfrac{2x+2y}{x+y}\)

⇒ \(\dfrac{x}{y}=2\) ⇒ x = 2y

Có :\(\dfrac{3y}{x-z}=2\) ⇔ 3y = 2x - 2z

      Mà : x = 2y ⇒ 3y = 2. 2y - 2z

                         ⇔ 3y = 4y - 2z

                         ⇔ 2z = y

Ta có:A=\(\dfrac{1}{3}+\dfrac{1}{6}+\dfrac{1}{10}+...+\dfrac{1}{561}\)

            =\(\dfrac{1}{1.3}+\dfrac{1}{2.3}+\dfrac{1}{2.5}+...+\dfrac{1}{17.33}\)

 ⇒\(\dfrac{1}{2}A=\dfrac{1}{1.2.3}+\dfrac{1}{2.3.2}+\dfrac{1}{2.5.2}+...+\dfrac{1}{17.33.2}\)

            =\(\dfrac{1}{2.3}+\dfrac{1}{3.4}+\dfrac{1}{4.5}+...+\dfrac{1}{33.34}\)

            =\(\dfrac{3-2}{2.3}+\dfrac{4-3}{3.4}+\dfrac{5-4}{4.5}+...+\dfrac{34-33}{33.34}\)

            =\(\dfrac{1}{2}-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{5}+...+\dfrac{1}{33}-\dfrac{1}{34}\)

             =\(\dfrac{1}{2}-\dfrac{1}{34}\)

              =\(\dfrac{8}{17}\)

⇒A=\(\dfrac{8}{17}:\dfrac{1}{2}=\dfrac{16}{17}\)

Vì \(\dfrac{16}{17}>\dfrac{8}{9}\) ⇒ A \(>\dfrac{8}{9}\)

a) 1 giờ máy A bơm được :      1:15=\(\dfrac{1}{15}\) (phần bể)

    1 giờ máy B bơm được :      1 : 18=\(\dfrac{1}{18}\)(phần bể)

1 giờ cả hai máy bơm bơm được :  \(\dfrac{1}{15}+\dfrac{1}{18}=\dfrac{11}{90}\)(phần bể)

b) 6 giờ hai máy A và B bơm được :\(\dfrac{11}{90}.6=\dfrac{11}{15}\)(phần bể)

3 giờ máy C bơm được là : \(1-\dfrac{11}{15}=\dfrac{4}{15}\)(phần bể)

1 giờ máy C bơm được là : \(\dfrac{4}{15}:3=\dfrac{4}{45}\)(phần bể)

Vậy máy C bơm một mình trong \(1:\dfrac{4}{45}=\dfrac{45}{4}\)=11 giờ 25 phút thì đầy bể