Read the following passage and mark the letter A, B, C or D on your answer sheet to indicate the correct word or phrase that best fits each of the numbered blanks from 25 to29. Fill in the appropriate word in question 29

A recent (25) _______ of a language learning magazine has consulted a number of experts in the field of second language acquisition. Their advice may prove invaluable for those considering a language course. One suggestion is that you access whether you are likely to be successful (26)_______ learning a language. Did you enjoy studying language at school, for example? Do you have enough time to learn a language ? The major cost will be your own time and effort. Therefore, you must make sure that course on offer leads to a recognized qualification. Also, be realistic in your (27)_______. If you don’t set achievable aims, you are more likely to give up. Don’t be deceived into thinking that the most expensive courses are the best. Shop around to get the best possible value for money. You should also bear in your mind that the quicker you learn a language, the more quickly you forget it. Sandra Miller, a French teacher, tried to teach herself German by enrolling on a crash course. Already fluent in four languages and with a sound knowledge of teaching methodology her chances of (28)_______ progress were high. Three years on she remembers very little. She feels her biggest mistake was not to follow up her first experience. "I think the teacher’s role is important. It's so nice to have somebody give you a/an (29)_______”.

A. help

B. encouragement

C. hand

D. aid

xét f(x)=ax^2 cộg bx cộg c 
f(x)-f(x-1)=x 
<=>2ax-(a-b)=x 
vì phân tích trên là duy nhất suy ra a=b=1/2 
nên f(x)=(x^2 cộng x)/2 cộg c (c là hằg số) 
cho x=0,1,2,...n rồi cộng lại ta đc: 
f(n)-f(0)=1 cộng 2 cộng...cộg n 
<=>(x^2 cộg x)/2=1 cộg 2 cộg...cộng n. 

lưu ý:từ bài này ta có thể suy ra cách tính tổng của một số dãy số.