SOLUTION: The seventh term of an arithmetic sequence is 25. Its first, third, and 21st
term form a geometric sequence. Find the first term and the common
difference of the sequence
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-> SOLUTION: The seventh term of an arithmetic sequence is 25. Its first, third, and 21st
term form a geometric sequence. Find the first term and the common
difference of the sequence
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Question 1187650: The seventh term of an arithmetic sequence is 25. Its first, third, and 21st
term form a geometric sequence. Find the first term and the common
difference of the sequence Answer by greenestamps(13198) (Show Source):
If the first term is a and the common difference is d, then
3rd term: a+2d
7th term: a+6d
21st term: a+20d
The given information is
[1] (the 7th term is 25)
[2] (the 1st, 3rd, and 21st term form a geometric sequence)
Working with [2] will give you an expression for d in terms of a; substituting the result into [1] will give you the answer. I'll get you started; the rest is straightforward.
