Find the sum of the first 21 terms of the sequence 5, 9, 13, 17, 21, ... a. 945 c. 947 b. 944 d. 946

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asked Sep 9, 2019 8.1k views

2 Answers

Zhongyu Kuang · Answer 1 · 2019-09-14T07:22:58+0000

See how the arithmetic sequence is made:
5, 5+4=9, 9+4=13, 13+4=17, ...
So the 21th term will be:
5( first term)+20( n-1 term)•4= 85
And the sum of the first 21 is :
21(5+85)/2= 21•45= 945

Find the sum of the first 21 terms of the sequence 5, 9, 13, 17, 21, ... a. 945 c. 947 b. 944 d. 946

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