tailieunhanh - Master the Gre 2010 - Part 24

Peterson's Master the Gre 2010 - Part book includes 9 full-length practice tests , thorough reviews of every section on the exam, and expert tips and strategies from a test prep pro. GRE (Graduage Record Examination General Test) is a commercially-run standardized test and an admission requirement for many graduate schools in USA and other English-speaking countries. GRE can earn you between 200-800 points. | Chapter 9 Math Review Number Forms Relationships and Sets 213 GEOMETRIC SEQUENCES In a geometric sequence of numbers each term is a constant multiple of the preceding one in other words the ratio between any term and the next one is constant. The multiple or ratio might be obvious by examining the sequence. For example In the geometric sequence 2 4 8 16 . you can easily see that the constant multiple is 2 and the ratio of each term to the next is 1 2 . In the geometric sequence 1 3 9 27 . . . you can easily see that the constant multiple is 3 and the ratio of each term to the next is 1 3 . Once you know the multiple or ratio you can answer any question asking for an unknown term or for either the sum or the average of certain terms. -32 -4 2 4 64 25. In a geometric sequence each term is a constant multiple of the preceding one. If the third and fourth numbers in the sequence are 8 and 16 respectively what is the first term in the sequence A B C D E The correct answer is C . The constant multiple is 2. But since you need to work backward from the third term 8 apply the reciprocal of that multiple twice. The second term is 8 4. The first term is 4 2 2. 26. In a geometric sequence each term is a constant multiple of the preceding one. What is the sum of the first four numbers in a geometric sequence whose second number is 4 and whose third number is 6 A 16 B 19 1 C 222 2 D 213 E 20 You can t calculate the average of terms in a geometric sequence by averaging the first and last term in the sequence The progression is geometric not arithmetic. You need to add up the terms then divide by the number of terms. The correct answer is D . The constant multiple is 2. In other words the ratio of each term to the next is 2 3. Since the second term is 4 the first term is 4 X . Since the third term is 6 the fourth term is 6 X or 9. The sum of 3 3 2 2 the four terms 1 4 6 9 21 33 You can also solve geometric sequence problems by applying a special formula. But you ll need to .