Sn=................................. and n=.................... if a=21,d=21,an=840?
Answers
Answered by
1
Step-by-step explanation:
an=840
21*n=840
n=40
sn=?????
Answered by
0
Answer:
Step-by-step explanation:
sn= ?
n= ?
a=21
d= 21
an= 840
an = a + (n-1)d
840 = 21 + (n-1) 21
840- 21 = (n-1) 21
819 =(n-1) 21
819/21 = n - 1
39 = n - 1
39 - 1 = n
38 = n
Sn = n/2 x ( a + an)
Sn = 38/2 x (21 + 840)
Sn = 38/ 2 x 861
Sn = 19 x 861
Sn = 16359
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