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How many terms are in the arithmetic sequence 5, 1, −3, . . . , −111?

Hint: an = a1 + d(n − 1), where a1 is the first term and d is the common difference.
 

 Jan 15, 2019
 #1
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5+(n-1)(4)=-111, (n-1)(4)=-116, n-1=-29, so n=30, 30 terms. smiley

 Jan 15, 2019
edited by tertre  Jan 16, 2019
 #2
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[L - F] / D + 1 = N, where L=Last term, F =First term, D=Common difference, N =Number of terms.

 

[-111 - 5] / - 4 + 1 = N

[-116] / -4 + 1       = N

29 + 1                   = N

N = 30 - Number of terms.

 Jan 15, 2019

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