Answers

2016-07-04T22:04:19+08:00
The multiples of 4 between 1 and 81 form an arithmetic sequence with first term 4 and last term 80. Finding the nth term:
80 = 4 + (n - 1)4
76 = (n - 1)4
19 = n - 1
n = 20

Solving for the sum:
Sn = 20/2 * (4 + 80)
Sn = 10(84)
Sn = 840
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2016-07-06T13:12:54+08:00
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