Bạn có thể chuẩn bị cho kỳ thi SAT khi đang học lớp 9,10,11 hoặc 12, và chuẩn bị càng sớm, cơ hội đạt điểm cao sẽ càng lớn. Phần khó nhất đối với người luyện thi SAT là phần Đọc và Viết. Hai phần này đòi hỏi người học có trí nhớ tốt và khả năng phân tích sâu sắc. Đối với sinh viên Việt Nam, phần toán lại là phần tương đối dễ và có nhiều khả năng dành điểm cao nếu bạn tập trung ôn luyện tốt hai kỹ năng Viết và Đọc hiểu. . | Averages 105 1. SIMPLE AVERAGE Most students are familiar with the method for finding an average and use this procedure frequently during the school year. To find the average of n numbers find the sum of all the numbers and divide this sum by n. Example Find the average of 12 17 and 61. Solution 12 17 61 3 90 30 When the numbers to be averaged form an evenly spaced series the average is simply the middle number. If we are finding the average of an even number of terms there will be no middle number. In this case the average is halfway between the two middle numbers. Example Find the average of the first 40 positive even integers. Solution Since these 40 addends are evenly spaced the average will be half way between the 20th and 21st even integers. The 20th even integer is 40 use your fingers to count if needed and the 21st is 42 so the average of the first 40 positive even integers that range from 2 to 80 is 41. The above concept must be clearly understood as it would use up much too much time to add the 40 numbers and divide by 40. Using the method described it is no harder to find the average of 100 evenly spaced terms than it is of 40 terms. In finding averages be sure the numbers being added are all of the same form or in terms of the same units. To average fractions and decimals they must all be written as fractions or all as decimals. Example Find the average of 87 2 4 and .6 Solution Rewrite each number as a decimal before adding. .875 .25 .6 3 .575 106 Chapter 7 Exercise 1 Work out each problem. Circle the letter that appears before your answer. r 3 1. Find the average of 4 and 80 . A .72 B .75 C .78 D .075 E .073 2. Find the average of the first 5 positive integers that end in 3. A 3 B 13 C 18 D 23 E 28 3. The five men on a basketball team weigh 160 185 210 200 and 195 pounds. Find the average weight of these players. A B C D E 190 192 195 198 180 4. Find the average of a 2a 3a 4a and 5a. A 3a5 B 3a C D E 3 5.
