I can't seem to figure out a problem that deals with arithmetic sequencing. This is the question: The 5th term in an arithmetic sequence is 1/2, and the 20th term is 7/8. Find the first three terms of the sequence. I attempted this problem with the formula: An = a + (n-1)d (where the n represents the nth term, a is the first term, and d represents the common difference) I keep getting -9.5 for the first number, and then 3/120 as the common difference between the numbers. But as I have figured it, the sequence is getting greater and greater, and my data does not go with the terms given. Please help!
Also, a similar question dealing with the same kind of problem is this: The sum of the first 12 terms in an arithmetic sequence is 156. What is the sum of the first and twelfth terms? Here I tried using the formula: Sn = n/2 [2a + (n-1)d]
(where n is the number of terms, a is the first term, and d is again the common difference). I have two unknowns and I don't know how to find even one!! Help!
For the first problem I used the same procedure as you did.
7/8 = a + 19d
For the second problem, when n = 12,