Hi frnds,
I have a doubt in one question of maths so please help me out
If the sum of the first 7 term of an A.P is 119 and that of the first 17 terms is 714,then find the sum of its first n terms..
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heya,..
this is so simple question.
S7=7/2{2a+(n-1)}d
S7=7/2(2a(2a+6d)
119×2/7=2a+6d
34=2a+6d
17=a+3d________----------1)
and again.
S17=17/2(2a+{17-1}d
714={17/2(16d)
714×2/17=2a+16d
84=2a+16d
42=a+8d-------_______2)
now from 1 ) and 2) we get .
d=5 and a =2 .
now .
Sn=n/2{2a+(n-1)d}
Sn={n/2(2×(2+n-1)}
Sn=5n^2-n/2 Ans .
hope it help you .
@rajukumar
this is so simple question.
S7=7/2{2a+(n-1)}d
S7=7/2(2a(2a+6d)
119×2/7=2a+6d
34=2a+6d
17=a+3d________----------1)
and again.
S17=17/2(2a+{17-1}d
714={17/2(16d)
714×2/17=2a+16d
84=2a+16d
42=a+8d-------_______2)
now from 1 ) and 2) we get .
d=5 and a =2 .
now .
Sn=n/2{2a+(n-1)d}
Sn={n/2(2×(2+n-1)}
Sn=5n^2-n/2 Ans .
hope it help you .
@rajukumar
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