Ta có : ( x + x + ... + x ) + ( 3 + 4 + 5 + ... + 22 ) = 450

Ta thấy : 3 + 4 + 5 + ... + 22 là dãy số cách đều 1 đơn vị và có số số hạng là : ( 22 - 3 ) : 1 + 1 = 20 ( số ) và tương ứng với 20 lần x . Tổng của dãy là : ( 22 + 3 ) x 20 : 2 = 250 

Thay vào ta được : 20x + 250 = 450

                             20x = 200

=>                            x = 10

B1: Chọn đáp án và gthich tại sao lại dùng thì đấy

1. The thief had stolen/ had been stealing cars for years before he was caught.

2.I was exhausted because i had painting/had painted all day.

3. They had been eating/had eaten all the pizza before i got there.

4. I had done/had been doing the housework all day, so i decided to take a break.

5.By the time we got to school ,the teacher had been/had been being there for twenty minutes.

6.I couldn't believe it had been snowing/had snowed all night until i saw how white everything was.

7.Fortunately, the plane had been landing/had landed before the engine caught fire.

8.The decorator had painted/had been painting all the walls before Dad got home.

P/s:Tất cả các câu đều hầu nư chia theo quá khứ hoàn thành,diễn tả 1 sự việc xảy ra trước 1 sự việc xác định trong quá khứ.

S + had + VII

1 :she / be / teacher / english / since 1995

=>She has been teacher of English since 1995.

2: she/ usually / go / school / bike

=>She usually goes to school by bike.

3: we / not see / each other / for 5 months

=>We haven't seen each other for 5 months.

4 :what / you / do / if / you / be / me ?

=>What would you do if you were me?

5 :mr.bill / used / go / swimming / when /he / young

=>Mr.Bin used to go swimming when he was young.

1; we ....go swimming together when he ....... in the village

a ;used to / lived

b; used to/ live

c ;are used to /live

d ;get used to / lived

2 ;my brother got wet ........ he did not bring the umbrella with him a; but b ;so c; because d; and

3 ;mai made a big beatiful cake ........................ her mother’ birthday a ;by b; in c; at d; on

4 ;they say that solar energy does not cause

a; pollution b; pollute c ;polluted d; pollutant

5; peter fell over while he ........................... basketball

a; is playing b; was playing c played d plays

6; the people ............live in england speak english

a; whom b; which c; who d ;whose

7 be .................! he is looking at you

a carefully

b care

c careful

d carefulessly

8;according to the weather .............., it will be raining tonight

a forecastingB: forecasted forecaster d forecast

9;jane speaks english ........than you do

a more fluently

b more fluent

c more faster

d more better

10;he made an interesting speech in the opening ......................... a competition b ceremony c preparation d solidarity

11 due to the bad weather , the flight to manchester was..........

a taken off b cut down c turned down d put off

12 nam : congratulations on your success ! hao : .....

a you have welcome

b no , thanks

c that is very kind of you

d yes , of course

Cartoon film have very few limits. If you can draw something, you can make it move on the cinema screen. The use(19) new ideas and advanced computer programs means that cartoons are becoming exciting again for people of all ages

By the end of the 1970s,the cinema world had decided that cartoons were only for children.

But soon afterward, one or two directors had some orginal new ideas. They proved that it was possible to make film in which both adults and children could (20) the fun.

However, not (21) cartoon film was successful. The Black Cauldron, for example, failed, mainly because it was too (22) for children and too childish for adults. Directors learnt form this mistake, and the film companies began to make large amounts of money again

Question 19:A.with B.for C.of D. by

Question 20:A.mix B.share C.divide D.add

Question 21:A.both B.every C.any D.each

Question 21:A.fearful B.afraid C.nervous D.frightening