Download Arithmetic and Geometric Sequences and more Study Guides, Projects, Research Calculus for Engineers in PDF only on Docsity! Arithmetic and Geometric Sequences A sequence is a list of numbers or objects, called terms, in a certain order. In an arithmetic sequence, the difference between one term and the next is always the same. This difference is called a common difference. The common difference is added to each term to get the next term. 2, 5, 8, 11, 14, … This is an increasing arithmetic sequence with a common difference of 3. 32, 26, 20, 14, 8, … This is a decreasing arithmetic sequence with a common difference of –6. Example: What are the next three terms in the sequence? 1, 5, 9, 13, … I can see that this is an arithmetic sequence with a common difference of 4. To get the next three terms, add 4 to 13 which equals 17, the next term in the sequence. Then add 4 to 17 to get the next term to get 21, etc. So the next three terms are 17, 21, and 25. Use the following formula to find any term of an arithmetic sequence. 1 ( 1)na a n d= + − an = the term in the sequence you are trying to find (n represents the desired term number) a1 = the first term in the sequence d = the common difference Example: What is the 10th term of the following sequence? 1, 5, 9, 13, … 10 1 (10 1)4 1 9 4 1 36 37a + − = + ⋅ = + == So the 10th term of this sequence is 37. Example: What is the 12th term of the following sequence? 34, 31, 28, 25, 22, … 12 34 (12 1)( 3) 34 11( 3) 34 ( 33) 1a + − − = + − = + − == The 12th term of this sequence is 1. Practice: 1. Find the next three terms: 3, 10, 17, 24, 31, _____, _____, _____ 2. Find the 25th term: 53, 50, 47, 44, 41, … ________ 3. Find the 20th term: 25, 40, 55, 70, 85, … ________ 4. Find the 75th term: 88, 81, 74, 67, 60, … ________ A geometric sequence is a sequence of numbers where the ratio of consecutive terms is constant. This ratio is called the common ratio (r). Sometimes the terms of a geometric sequence get so large that you may need to express the terms in scientific notation rounded to the nearest tenth. 2, 6, 18, 54, … This is an increasing geometric sequence with a common ratio of 3. 1,000, 200, 40, 8, … This is a decreasing geometric sequence with a common ratio or 0.2 or 5 1 . Example: What are the next three terms of the following sequence? 4, 20, 100, 500, … 500,625500,12 500,1252500 25005500 =⋅ =⋅ =⋅ The next three terms are 2,500, 12,500, and 62,500. Explicit sequences also have a formula for finding any term in a sequence. 1 ( 1) n na a r −= an = the term in the sequence you are trying to find (n represents the desired term number) a1 = the first term in the sequence r = the common ratio Example: Find the 7th term in the following sequence: 6, 18, 54, 162, … Finding the common ratio can be harder than finding the common difference. One way to find it is the divide each term by the term before it. 3618 =÷ , 31854 =÷ , 354162 =÷ So the common ratio is 3. (7 1) 6 7 6 3 6 3 6 729 4,374a −⋅ ⋅ ⋅ == = = So the 7th term of the sequence is 4, 374. Example: Find the 8th term in the following sequence: 96, 48, 24, 12, 6, … To find the common ratio, divide each term by the one before it. 2 19648 =÷ , 2 14824 =÷ , 2 12412 =÷ The common ratio is 2 1 . 3. Use your formula from question 2c to find the values of a7 and a20. 4. For the following geometric sequences, find a and r and state the formula for the general term. a) 1, 3, 9, 27, ... b) 12, 6, 3, 1.5, ... c) 9, -3, 1, ... 5. Use your formula from question 4c) to find the values of the a4 and a12 6. Find the number of terms in the following arithmetic sequences. Hint: you will need to find the formula for tn first! a) 2, 5, 8, ..... , 299 b) 9, 5, 1, ..... - 251. Answers: 1a) arithmetic d = 6 b) neither c) geometric r = 7 d) geometric r = 0.5 or r = ½ e) arithmetic d = 15 f) geometric r = -3 2a) a = -10; d=6; tn =6n-16 b) a = 10; d=-2; tn=-2n+12 c) a = 36; d=-5; tn= - 5n+41 3. t7=6; t20 = -59 4. a) a = 1; r = 3; tn = 1(3)n –1 b) a = 12; r = ;a c) a = 9; r = - 3; tn = 9(-3)n –1 5. t4 = -243 t12 = -177147 6. a) tn = 3n-1; n = 100 b) tn = -4n+13; n=66 General formula for a geometric series: 1) Find the designated sum of the arithmetic series a) 𝑆14 of 3+7+11+15+⋯ b) 𝑆11 of −13−11−9−7−⋯ c) 𝑆9 of 22+20+18+16+⋯ d) 𝑆35 of −2−5−8−11−⋯ 2) Determine the sum of each arithmetic series a) 6+13+20+⋯+69 b) 4+15+26+⋯+213 c) 5−8−21−⋯−190 d) 100+90+80+⋯−100 Arithmetic and Geometric Series – Worksheet General formula for an arithmetic series: 3) Find the designated sum of the geometric series a) 𝑆7 of 4+8+16+32+⋯ b) 𝑆13 of 1−6+36−216+⋯ c) 𝑆17 of 486+162+54+18+⋯ 4) Determine 𝑆𝑛 for each geometric series d) 𝑆6 of 3+15+75+375+⋯ a) 𝑎=6, 𝑟=2, 𝑛=9 b) 𝑓 1 =2, 𝑟=−2, 𝑛=12 c) 𝑓 1 =729, 𝑟=−3, 𝑛=15 d) 𝑓 1 =2700, 𝑟=10, 𝑛=8 5) If the first term of an arithmetic series is 2, the last term is 20, and the increase constant is +2 … a) Determine the number of terms in the series b) Determine the sum of all the terms in the series 6) A geometric series has a sum of 1365. Each term increases by a factor of 4. If there are 6 terms, find the value of the first term.