The first term of a arithmetic sequence is 640 and the 8th term is 493. What is the common difference in the sequence

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The first term is usually represented by a, the next term is a+d and the nth term is a+(n-1)d.

So a=640 and the 8th term (n=8) is 493. So 493=640+7d and 7d=493-640=-147 therefore d=-147/7=-21.

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