If
y = 1 ∑ 1 1 1 y 4 + y y 4 + y 2 + 1 = b a .
Find the value of ( a + b ) × 1 1 1 .
Inspired by this .
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Nice Solution!
BTW, there is one more pattern. If we have to find the sum till n terms starting from 1 , the answer will always be n + 1 ( n + 1 ) 2 − 1 .
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Yea...It can be derived from here...I made that as well, but thought of explaining the entire one, so as to make it easy for everyone to understand!
Yes I did it this way, see in dispute.
[n(n+2)/n+1 ]=[a/b]
we have n=111 & n+2=113 & n+1=112
a=111
113= 12543 & b=112
so 111
(a+b)= 111*12655= 1404705
ya, I did it the same way... nice!!
Let's factor the argument of the summation.
y 4 + y y 4 + y 2 + 1
= y ( y + 1 ) ( y 2 − y + 1 ) ( y 2 + y + 1 ) ( y 2 − y + 1 ) (See Sophie Germain identity and sum of cubes)
= y 2 + y y 2 + y + 1
= 1 + y 2 + y 1
= 1 + ( y 1 − y + 1 1 ) (Partial fraction decomposition)
Now we can substitute this into the summation.
y = 1 ∑ 1 1 1 1 + y 1 − y + 1 1
= 1 1 1 + y = 1 ∑ 1 1 1 y 1 − y + 1 1
= 1 1 1 + 1 − 1 1 2 1 (Telescoping Series)
= 1 1 2 1 2 5 4 3
a + b = 1 2 6 5 5 , then 1 1 1 ( a + b ) = 1 4 0 4 7 0 5 .
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∑ y = 1 1 1 1 y 4 + y y 4 + y 2 + 1
Now, since, we know that: a 3 − b 3 = ( a − b ) ( a 2 + a b + b 2 )
PUTTING
a = y 2 , b = 1
WE GET : y 6 − 1 = ( y 2 − 1 ) ( y 4 + y 2 + 1 ) ⇒ ( y 4 + y 2 + 1 ) = ( y 2 − 1 ) y 6 − 1 -------1.
ALSO
y 4 + y = y ( y 3 + 1 ) -------2.
Using in the main equation, we need to find:
∑ y = 1 1 1 1 y ( y 3 + 1 ) ( y 2 − 1 ) y 6 − 1 ⇒ ∑ y = 1 1 1 1 y ( y 3 + 1 ) ( y 2 − 1 ) y 6 − 1 ⇒ ∑ y = 1 1 1 1 y ( y 3 + 1 ) ( y 2 − 1 ) ( y 3 − 1 ) ( y 3 + 1 ) ⇒ ∑ y = 1 1 1 1 y ( y 2 − 1 ) ( y 3 − 1 ) ⇒ ∑ y = 1 1 1 1 y ( y − 1 ) ( y + 1 ) ( y − 1 ) ( y 2 + y + 1 ) ⇒ ∑ y = 1 1 1 1 y ( y + 1 ) ( y 2 + y + 1 ) ⇒ ∑ y = 1 1 1 1 y ( y + 1 ) ( y + 1 ) 2 − y ⇒ ∑ y = 1 1 1 1 y y + 1 − ∑ y = 1 1 1 1 y + 1 1 ⇒ ∑ y = 1 1 1 1 1 + ∑ y = 1 1 1 1 y 1 − ∑ y = 1 1 1 1 y + 1 1
Now, since there is no fixed formula, we need to identify a pattern. See, by putting initial values, we can see that, the consecutive terms do get cancelled.
Since ∑ y = 1 1 1 1 1 = 1 1 1 , so the question is reduced to: 1 1 1 + ∑ y = 1 1 1 1 y 1 − ∑ y = 1 1 1 1 y + 1 1 ⇒ 1 1 1 + 1 − 2 1 + 2 1 − 3 1 + . . . . + 1 1 1 1 − 1 1 2 1 ⇒ 1 1 1 + 1 − 1 1 2 1 ⇒ 1 1 2 1 2 5 4 3 = b a ⇒ a + b = 1 2 6 5 5 ⇒ ( a + b ) × 1 1 1 = 1 4 0 4 7 0 5
Phew! Cheers!!:):)