No. 111!

Algebra Level 4

If

y = 1 111 y 4 + y 2 + 1 y 4 + y = a b \color{#69047E}{\huge{\displaystyle\sum_{y=1}^{111}\dfrac{y^4+y^2+1}{y^4+y}}=\dfrac{a}{b}} .

Find the value of ( a + b ) × 111 (a+b)\times 111 .

Inspired by this .


The answer is 1404705.

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3 solutions

y = 1 111 y 4 + y 2 + 1 y 4 + y \sum _{ y=1 }^{ 111 }{ \frac { { y }^{ 4 }+{ y }^{ 2 }+1 }{ { y }^{ 4 }+y } } \\

Now, since, we know that: a 3 b 3 = ( a b ) ( a 2 + a b + b 2 ) { a }^{ 3 }-{ b }^{ 3 }\quad =\quad (a-b)({ a }^{ 2 }+ab+{ b }^{ 2 })

PUTTING

a = y 2 , b = 1 a\quad =\quad { y }^{ 2 },\quad b\quad =\quad 1

WE GET : y 6 1 = ( y 2 1 ) ( y 4 + y 2 + 1 ) ( y 4 + y 2 + 1 ) = y 6 1 ( y 2 1 ) \quad \quad { y }^{ 6 }-1\quad =\quad ({ y }^{ 2 }-1)({ y }^{ 4 }+{ y }^{ 2 }+1)\\ \Rightarrow ({ y }^{ 4 }+{ y }^{ 2 }+1)\quad =\quad \frac { { y }^{ 6 }-1 }{ ({ y }^{ 2 }-1) } -------1.

ALSO

y 4 + y = y ( y 3 + 1 ) { y }^{ 4 }+y\quad =\quad y({ y }^{ 3 }+1) -------2.

Using in the main equation, we need to find:

y = 1 111 y 6 1 ( y 2 1 ) y ( y 3 + 1 ) y = 1 111 y 6 1 y ( y 3 + 1 ) ( y 2 1 ) y = 1 111 ( y 3 1 ) ( y 3 + 1 ) y ( y 3 + 1 ) ( y 2 1 ) y = 1 111 ( y 3 1 ) y ( y 2 1 ) y = 1 111 ( y 1 ) ( y 2 + y + 1 ) y ( y 1 ) ( y + 1 ) y = 1 111 ( y 2 + y + 1 ) y ( y + 1 ) y = 1 111 ( y + 1 ) 2 y y ( y + 1 ) y = 1 111 y + 1 y y = 1 111 1 y + 1 y = 1 111 1 + y = 1 111 1 y y = 1 111 1 y + 1 \quad \quad \sum _{ y=1 }^{ 111 }{ \frac { \frac { { y }^{ 6 }-1 }{ ({ y }^{ 2 }-1) } }{ y({ y }^{ 3 }+1) } } \\ \\ \Rightarrow \sum _{ y=1 }^{ 111 }{ \frac { { y }^{ 6 }-1 }{ y({ y }^{ 3 }+1)({ y }^{ 2 }-1) } } \\ \\ \Rightarrow \sum _{ y=1 }^{ 111 }{ \frac { { (y }^{ 3 }-1)({ y }^{ 3 }+1) }{ y({ y }^{ 3 }+1)({ y }^{ 2 }-1) } } \\ \\ \Rightarrow \sum _{ y=1 }^{ 111 }{ \frac { { (y }^{ 3 }-1) }{ y({ y }^{ 2 }-1) } } \\ \\ \Rightarrow \sum _{ y=1 }^{ 111 }{ \frac { { (y }-1)({ y }^{ 2 }+y+1) }{ y({ y }-1)(y+1) } } \\ \\ \Rightarrow \sum _{ y=1 }^{ 111 }{ \frac { ({ y }^{ 2 }+y+1) }{ y(y+1) } } \\ \\ \Rightarrow \sum _{ y=1 }^{ 111 }{ \frac { { (y+1) }^{ 2 }-y }{ y(y+1) } } \\ \\ \Rightarrow \sum _{ y=1 }^{ 111 }{ \frac { y+1 }{ y } } -\sum _{ y=1 }^{ 111 }{ \frac { 1 }{ y+1 } } \\ \\ \Rightarrow \sum _{ y=1 }^{ 111 }{ 1 } +\sum _{ y=1 }^{ 111 }{ \frac { 1 }{ y } } -\sum _{ y=1 }^{ 111 }{ \frac { 1 }{ y+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 = 1 111 1 = 111 \sum _{ y=1 }^{ 111 }{ 1 } =111 , so the question is reduced to: 111 + y = 1 111 1 y y = 1 111 1 y + 1 111 + 1 1 2 + 1 2 1 3 + . . . . + 1 111 1 112 111 + 1 1 112 12543 112 = a b a + b = 12655 ( a + b ) × 111 = 1404705 \quad \quad \quad 111+\sum _{ y=1 }^{ 111 }{ \frac { 1 }{ y } } -\sum _{ y=1 }^{ 111 }{ \frac { 1 }{ y+1 } } \\ \\ \Rightarrow \quad 111+1-\frac { 1 }{ 2 } +\frac { 1 }{ 2 } -\frac { 1 }{ 3 } +....+\frac { 1 }{ 111 } -\frac { 1 }{ 112 } \\ \Rightarrow \quad 111+1-\frac { 1 }{ 112 } \\ \Rightarrow \quad \frac { 12543 }{ 112 } \quad =\quad \frac { a }{ b } \\ \Rightarrow \quad a+b\quad =\quad 12655\\ \Rightarrow \quad (a+b)\quad \times \quad 111\quad =\quad 1404705\quad

Phew! Cheers!!:):)

Nice Solution!

BTW, there is one more pattern. If we have to find the sum till n n terms starting from 1 1 , the answer will always be ( n + 1 ) 2 1 n + 1 \dfrac{(n+1)^{2}-1}{n+1} .

Yash Singhal - 6 years, 4 months ago

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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!

A Former Brilliant Member - 6 years, 4 months ago

Yes I did it this way, see in dispute.

U Z - 6 years, 4 months ago
Hajar Aggad
Feb 21, 2015

[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!!

Raushan Sharma - 6 years, 3 months ago
Elijah L
Apr 13, 2020

Let's factor the argument of the summation.

y 4 + y 2 + 1 y 4 + y \displaystyle \frac{y^4+y^2+1}{y^4+y}

= ( y 2 + y + 1 ) ( y 2 y + 1 ) y ( y + 1 ) ( y 2 y + 1 ) \displaystyle = \frac{(y^2+y+1)(y^2-y+1)}{y(y+1)(y^2-y+1)} (See Sophie Germain identity and sum of cubes)

= y 2 + y + 1 y 2 + y \displaystyle = \frac{y^2+y+1}{y^2+y}

= 1 + 1 y 2 + y \displaystyle = 1 + \frac{1}{y^2+y}

= 1 + ( 1 y 1 y + 1 ) \displaystyle = 1 + \left(\frac{1}{y} - \frac{1}{y+1}\right) (Partial fraction decomposition)

Now we can substitute this into the summation.

y = 1 111 1 + 1 y 1 y + 1 \displaystyle \sum_{y=1}^{111} 1 + \frac{1}{y} - \frac{1}{y+1}

= 111 + y = 1 111 1 y 1 y + 1 \displaystyle = 111 + \sum_{y=1}^{111} \frac{1}{y} - \frac{1}{y+1}

= 111 + 1 1 112 \displaystyle = 111 + 1 - \frac{1}{112} (Telescoping Series)

= 12543 112 \displaystyle = \frac{12543}{112}

a + b = 12655 a + b = 12655 , then 111 ( a + b ) = 1404705 111(a+b) = \boxed{1404705} .

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