If in an A.P s1=6,s7=105,then sn:sn-3 is same as
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s1=1/2(2a) (a = first term)
S1= a = 6
a = 6
now, s7= 7/2(2a+(7-1)d)
s7 = 105 = 7/2(12+6d)
105x2/7 = 12+6d
15x2 = 12+6d
30-12 = 6d
18=6d
d=3
sn=n/2(2a+(n-1)d)
sn= n/2(12+(n-1)3) = n/2(12+3n-3)
sn= n/2(9+3n)....................(i)
Sn-3 = (n-3)/2 (2a+(n-3-1)d)
Sn-3 = (n-3)/2 (12+(n-4)3)
Sn-3 = (n-3)/2 (12+3n-12)
Sn-3 = (n-3)/2 (3n).............(ii)
(i):(ii)
Sn:Sn-3 = n/2(9+3n)÷[(n-3)/2 (3n)]
=> n(9+3n) ÷ 3n (n-3)
=> (9+3n)/3(n-3)
=>3(3+n)/3(n-3)
=> n+3:n-3 = Sn: Sn-3 for given values of question
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yes correct answer it helps me a lot
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