If the sum of first n term of the series 5,9,13,17.......is 275,then find n
Answers
Answered by
7
Step-by-step explanation:
Sn= n/2(2a+(n-1) d)
275= n/2(10+4n-4)
275= n/2(6+4n)
550= 6n+4n^2
2n^2+3n-275=0
D= b^2-4ac
= 9+2200
=2209
n= (-b+or-√D)/2a
n= (-3+or-47) 4
n= -(25/2) or 44/4=11
neglecting n= -25/2, we get n= 11
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Answered by
4
Answer:
Sn=n/2[2a+(n-1)d]
275=n/2[10+(n-1)4]
550=n[10+4n-4]
550=6n+4n^2
4n^2+6n-550=0
2n^2+3n-275=0
2n^2+25n-22n-275=0
n(2n+25)-11(2n+25)=0
(2n+25)(n-11)=0
either,
n=-25/2 or n=11
as no of terms cant be taken,so
n=11
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