a-2
d=8
Sn=90
find
n and an
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Answered by
1
Step-by-step explanation:
refer the attachment,it may be helpful
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Answered by
2
Step-by-step explanation:
an = a + (n-1) d
an = 2 + ( n -1 ) 8
= 2 + 8n -8
= -6 +8n ..... 1
Sn = n/2 [ 2a + (n-1) d]
= n/2 [ 2× 2 + ( n- 1) 8]
= n/2 [ 4 + 8n -8 ]
= n/2 [ -4 + 8n]
= n/2 ( 4 ) [ -1 + 2n]
2n [ -1 +2n]
= -2n + 4n^2
Sn = 90
-2n + 4n^2 = 90
-n + 2n^2 = 45
2n^2 - n - 45=0
2n ^2 - 10 n + 9n - 45 =0
2n ( n -5 ) + 9 ( n -5 ) =0
( n-5 ) ( 2n + 9)
n = 5, 2n + 9 = 0
n= 5 , n = -9/2
therefore, n= 5
substituting the value of n in equation 1
an = -6 +8×5
= -6 + 40
=34
therefore, an = 34, n= 5.
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