Consider an arithmetic progression:- 8 5 , 8 1 7 , 8 2 9 , 8 4 1 , ⋯ . How many terms of this AP are needed to give the sum of the terms as 4 2 9 5 ?
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I thought that it would be lengthy so I posted an alternative solution plz do have a look. Btw nice solution (+1)
Here we see that the first term of the AP(a) is 8 5 and the common difference(d) = 8 1 7 − 5 = 8 1 2 = 2 3 . Let the number of terms required be n .
S n = 2 n × ( 2 a + ( n − 1 ) d ) 4 2 9 5 = 2 n × ( 2 × 8 5 + ( n − 1 ) × 2 3 ) 4 2 9 5 = 2 n × ( 4 5 + 2 3 n − 3 ) 4 2 9 5 = 2 n × 4 5 + 6 n − 6 4 2 9 5 = 8 6 n 2 − n 4 2 9 5 × 8 = 6 n 2 − n 5 9 0 = 6 n 2 − n 6 n 2 − n − 5 9 0 = 0
Applying quadratic formula:-
n
=
2
×
6
−
(
−
1
)
±
(
−
1
)
2
−
4
×
6
×
−
5
9
0
n
=
1
2
1
±
1
+
1
4
1
6
0
n
=
1
2
1
±
1
4
1
6
1
n
=
1
2
1
±
1
1
9
n
=
1
2
1
+
1
1
9
(
or
)
1
2
1
−
1
1
9
n
=
1
2
1
2
0
(
or
)
=
1
2
−
1
1
8
n
=
1
0
(
or
)
6
−
5
9
But, number of terms can never be negative. So, n = 6 − 5 9 . So, n = 1 0 .
Alternatively:-
Observe that the numerators of the terms are in AP with first term(a) =
5
and common difference
1
2
. Why? Because the actual common difference
2
3
can be written as
8
1
2
. So denominator remains the same but numerator increases by
1
2
.
The sum given whihc is
4
2
9
5
can be written as
8
5
9
0
. So, now we just have to find how much terms of the AP
5
,
1
7
,
2
9
,
4
1
,
⋯
give the sum as
5
9
0
. Let
m
terms be required.
S
m
=
2
m
×
(
2
a
+
(
m
−
1
)
d
)
5
9
0
=
2
m
×
(
2
×
5
+
(
m
−
1
)
×
1
2
)
5
9
0
=
2
m
×
2
(
5
+
6
(
m
−
1
)
)
5
9
0
=
m
(
5
+
6
m
−
6
)
5
9
0
=
6
m
2
−
m
6
m
2
−
m
−
5
9
0
=
0
6
m
2
−
6
0
m
+
5
9
m
−
5
9
0
=
0
6
m
(
m
−
1
0
)
+
5
9
(
m
−
1
0
)
=
0
(
m
−
1
0
)
(
6
m
+
5
9
)
=
0
m
−
1
0
=
0
(
or
)
6
m
+
5
9
=
0
n
=
1
0
(
or
)
6
−
5
9
But, number of terms can never be negative. So, n = 6 − 5 9 . So, n = 1 0 .
There is a Typo is the alternative answer
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Oops I corrected it. It sometimes take ages to track down a minor mistake in LaTeX.
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AP is the easiest Math topic.
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@Abhiram Rao – Correct and it is also the most beautiful one.
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I totally Like your Solution @Ashish Siva , but we can Reduce the calculation .
We have an A.P of Common Difference ( d ) = 8 1 7 − 8 5 = 8 1 2 and First Term ( a ) = 8 5 . Let the Required number of terms be n
S n = 4 2 9 5 = 2 n ( 4 5 + ( n − 1 ) 4 6 ) 5 9 0 = n ( 5 + 6 n − 6 ) n ( 6 n − 1 ) = 5 9 0 n ( 6 n − 1 ) = 1 0 ( 6 × 1 0 − 1 ) n = 1 0
Since d > 0 , the quadratic will only give us only 1 +ve Solution