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the 6th term of an arithmetic sequence is 15 and the 18th term is -9 find d&a​

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Answer:

a1 = 25

d = -2

Step-by-step explanation:

An = A1 + ( n - 1 ) d

-9 = 15 + ( 13 - 1 ) d

-9 = 15 + 12d

-9-15 = 12d

12d = -24

d = -2

-9 = a1 + ( 18 - 1 ) x -2

-9 = a1 + 17 x -2

-9 = a1 - 34

a1 = 34 -9

a1 = 25

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