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Algebra Level 4

n = 1 2015 [ ( 1 ) n ( n 2 + n + 1 n ! ) ] \large \sum_{n=1}^{2015} \left [ (-1)^n \left ( \frac { n^2+n+1}{n!} \right ) \right ]

If the summation equals to a b c ! -a - \frac {b}{c!} for positive integers a , b , c a,b,c

What is the minimum value of a + b + c a+b+c ?


The answer is 4032.

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3 solutions

Sandeep Rathod
Feb 13, 2015

n = 1 2015 ( 1 ) n ( n 2 n ! + n + 1 n ! ) \sum_{n=1}^{2015} (-1)^n (\dfrac{n^2}{n!} + \dfrac{n+1}{n!})

n = 1 2015 ( 1 ) n ( n ( n 1 ) ! + n + 1 n ! ) \sum_{n=1}^{2015} (-1)^n (\dfrac{n}{(n-1)!} + \dfrac{n+1}{n!})

1 2 1 ! + 2 1 ! + 3 2 ! 3 2 ! 4 3 ! + . . . ( 1 ) 2015 ( 2015 2014 ! + 2016 2015 ! ) -1 - \dfrac{2}{1!} + \dfrac{2}{1!} + \dfrac{3}{2!} - \dfrac{3}{2!} - \dfrac{4}{3!} + ... (-1)^{2015} (\dfrac{2015}{2014!} + \dfrac{2016}{2015!})

We can see that alternate terms cancel each other

1 2016 2015 ! -1 - \dfrac{2016}{2015!}

a + b + c = 4032 a + b + c = 4032

Math man toogle the latex and copy the latex and edit your question because the 1 you have written looks like 2

sandeep Rathod - 6 years, 4 months ago

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OK , it is 4032:)

math man - 6 years, 4 months ago

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thanks for the help , please do it quickly so others don't confuse and from toogling other's people latex you can learn to write in latex

sandeep Rathod - 6 years, 4 months ago

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@Sandeep Rathod ok , its done :)

math man - 6 years, 4 months ago
Lu Chee Ket
Feb 14, 2015

My break is more numerical for numerical analysis. Not as good as f (n) + f (n + 1).

Lu Chee Ket - 6 years, 3 months ago

Did exact same!!! Gd solution...

Rushikesh Joshi - 6 years, 3 months ago
Math Man
Feb 13, 2015

How to do it the other way

notice that if we switch 2015 to 1,3,5,7.... we see

when 2015 is 1 it is 1 2 1 ! -1-\dfrac{2}{1!}

when 2015 is 3 it is 1 4 3 ! -1-\dfrac{4}{3!}

.....

thus when 2015 is 2015 it is 1 2016 2015 ! ) -1-\dfrac{2016}{2015!})

so, a=1,b=2016,c=2015,a+b+c= 4032

So in a way you are doing mathematical induction . Good

for writing in fraction use - \dfrac{2}{1!} when encased in brackets , it will give - 2 1 ! \dfrac{2}{1!}

sandeep Rathod - 6 years, 4 months ago

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ok , and thanks for the advice

my LATEX is bad ._.

math man - 6 years, 4 months ago

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toogle latex is made so that one can learn latex. thanks

sandeep Rathod - 6 years, 4 months ago

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@Sandeep Rathod ok yw. sir

math man - 6 years, 4 months ago

/dfrac{2}{5}

Raunak Agrawal - 4 years, 3 months ago

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