T n = cot − 1 2 + cot − 1 8 + cot − 1 1 8 + ⋯ cot − 1 2 n 2
n → ∞ lim T n = γ α π where α and γ are coprime. α + 4 γ = ?
This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try
refreshing the page, (b) enabling javascript if it is disabled on your browser and,
finally, (c)
loading the
non-javascript version of this page
. We're sorry about the hassle.
Neat solution as usual! =D
yeah solved it !!! @Rishabh Cool i guess you love summation ???
Same solution @Rishabh Cool .
S = n → ∞ lim k = 1 ∑ n cot − 1 ( 2 k 2 )
S
=
n
→
∞
lim
k
=
1
∑
n
tan
−
1
(
2
k
2
1
)
=
n
→
∞
lim
k
=
1
∑
n
tan
1
(
1
+
(
2
k
+
1
)
(
2
k
−
1
)
(
2
k
+
1
)
−
(
2
k
−
1
)
)
∴
S
=
n
→
∞
lim
k
=
1
∑
n
tan
−
1
(
2
k
+
1
)
−
tan
−
1
(
2
k
−
1
)
∴
S
=
n
→
∞
lim
tan
−
1
(
3
)
−
tan
−
1
(
1
)
+
tan
−
1
(
5
)
−
tan
−
1
(
3
)
+
…
tan
−
1
(
2
n
+
1
)
−
tan
−
1
(
2
n
−
1
)
=
n
→
∞
lim
tan
−
1
(
2
n
+
1
)
−
tan
−
1
(
1
)
=
2
π
−
4
π
=
4
π
α
+
4
γ
=
1
+
4
⋅
4
=
1
7
Problem Loading...
Note Loading...
Set Loading...
T n = = = = r = 1 ∑ n tan − 1 ( 2 r 2 1 ) r = 1 ∑ n tan − 1 ( 4 r 2 2 ) r = 1 ∑ n tan − 1 ( 1 + ( 2 r + 1 ) ( 2 r − 1 ) ( 2 r + 1 ) − ( 2 r − 1 ) ) r = 1 ∑ n tan − 1 ( 2 r + 1 ) − tan − 1 ( 2 r − 1 )
( A T e l e s c o p i c S e r i e s )
T n = tan − 1 ( 2 n + 1 ) − tan − 1 ( 1 ) = tan − 1 ( 2 n + 1 ) − 4 π
∴ n → ∞ lim T n = 2 π − 4 π = 4 π
∴ 1 + 4 ( 4 ) = 1 7