Find the the sum of the first 21st term of the arithmetic sequence 3,10,17...
With solution po
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Find the the sum of the first 21st term of the arithmetic sequence 3,10,17...
With solution po
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Given:
a1 = 3 n = 21 d = 7
Sn = n/2 [2a1 + (n - 1) d]
S21 = 21/2 [2(3) + (21 - 1) 7]
S21 = 21/2 [6 + (20) 7]
S21 = 21/2 [6 + 140]
S21 = 21/2 [146]
S21 = 21/2 [146/1]
S21 = 3066/2
S21 = 1533
Answer:
Given:
a1=3
n=21
d=7
Sn=n/2[2a1+(n-1)d]
S sub 21=21/2[2(3)+(21-1)7]
=10.5[6+(20)7]
=10.5(6+140)
=10.5(146)
=1,533