An Interesting Series

Algebra Level 2

For what value of n n is

1 ( 1 ! ) + 2 ( 2 ! ) + 3 ( 3 ! ) + . . . + 2014 ( 2014 ) ! = n ! 1 1(1!)+2(2!)+3(3!)+...+2014(2014)!=n!-1


The answer is 2015.

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5 solutions

Vighnesh Raut
Sep 5, 2014

1.1 ! + 2.2 ! + 3.3 ! + . . . . . + n . n ! = ( 2 1 ) 1 ! + ( 3 1 ) 2 ! + ( 4 1 ) 3 ! + . . . + [ ( n + 1 ) 1 ] n ! = 2 ! 1 ! + 3 ! 2 ! + 4 ! 3 ! + . . . . . . + ( n + 1 ) ! n ! = ( n + 1 ) ! 1 ! \quad 1.1!+2.2!+3.3!+.....+n.n!\\ =(2-1)1!+(3-1)2!+(4-1)3!+...+[(n+1)-1]n!\\ =2!-1!+3!-2!+4!-3!+......+(n+1)!-n!\\ =(n+1)!-1!

H e r e , t h e v a l u e o f n i s 2014. S o , t h e a n s w e r i s 2015. Here,\quad the\quad value\quad of\quad n\quad is\quad 2014.\\ So,\quad the\quad answer\quad is\quad 2015.

Nice solution...

Anik Mandal - 6 years, 9 months ago

great answer

Ajay Aggarwal - 6 years, 8 months ago

Great solution.

Aswanth Krishnan - 6 years, 8 months ago

pretty solution.

Panya Chunnanonda - 6 years, 7 months ago

gddd answer

Shivam Rana - 6 years, 8 months ago
Nick Lee
Sep 5, 2014

Here is another way. 1(1!)+2(2!)+..+2014(2014!)={2(1!)+3(2!)+...+2015(2014!)} -{1!+2!+...+2014!}={2!+3!+...+2015!}-{1!+2!+...+2014!}=2015!-1

찬홍 민
Dec 25, 2014

Great solution! Very simple.

Nick Lee - 6 years, 5 months ago
Akhilesh Singh
Sep 3, 2014

Just hit n trial!.consider first two terms which is 1+4=5=3!-1.Now consider first three 1+4+18=23=4!-1. The last no of the series is 2014,therefore the ans is 2015

That's one way. But, there is a more of a logical way to solve this :D

Nick Lee - 6 years, 9 months ago
Abdul Lah
Sep 29, 2014

by mathematical induction we can prove that 1.1! + 2.2!+..............+ n.n! = (n+1)!-1 hence here n = 2014 so n +1 = 2015

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