Cubic summation 2

Algebra Level 5

x = 1 3 x 2 + 12 x + 16 ( x ( x + 1 ) ( x + 2 ) ( x + 3 ) ( x + 4 ) ) 3 = 1 4 ( a ! ) b \sum _{ x=1 }^{ \infty } \frac { { 3x }^{ 2 }+12x+16 }{ { (x(x+1)(x+2)(x+3)(x+4)) }^{ 3 } } =\quad \frac { 1 }{ 4{ (a!) }^{ b } } Compute b a b-a if b b and a a are positive integers.


The answer is -1.

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1 solution

Shreyash Rai
Jan 12, 2016

x = 1 3 x 2 + 12 x + 16 ( x ( x + 1 ) ( x + 2 ) ( x + 3 ) ( x + 4 ) ) 3 the above expression can be rewritten as 1 ( x + 4 ) 3 ( x ) 3 ( x ( x + 1 ) ( x + 2 ) ( x + 3 ) ( x + 4 ) ) 3 1 4 w h i c h w i l l g i v e u s 1 4 1 1 ( x ( x + 1 ) ( x + 2 ) ( x + 3 ) ) 3 1 ( ( x + 1 ) ( x + 2 ) ( x + 3 ) ( x + 4 ) ) 3 t h u s w e c a n s e e t h a t i t w o u l d f o r m a t e l e s c o p i c s u m w h e r e a l l t h e t e r m s e x c e p t t h e f i r s t o n e w i l l g e t c a n c e l l e d o u t . t h u s o u r a n s w e r w o u l d b e 1 ( 1 2 3 4 ) 3 1 4 = 1 ( 4 ! ) 3 1 4 t h u s o u r a w o u l d b e 4 a n d b w o u l d b e 3. t h u s t h e f i n a l a n s w e r w o u l d b e B A = 3 4 = 1. \sum _{ x=1 }^{ \infty } \frac { { 3x }^{ 2 }+12x+16 }{ { (x(x+1)(x+2)(x+3)(x+4)) }^{ 3 } } \\ \quad \\ \text{the above expression can be rewritten as}\\ \\ \sum _{ 1 }^{ \infty }{ \frac { { (x+4) }^{ 3 }-{ (x) }^{ 3 } }{ { (x(x+1)(x+2)(x+3)(x+4)) }^{ 3 } } } *\frac { 1 }{ 4 } \\ \quad \\ which\quad will\quad give\quad us\\ \\ \frac { 1 }{ 4 } \sum _{ 1 }^{ \infty }{ \frac { 1 }{ { (x(x+1)(x+2)(x+3)) }^{ 3 } } } -\frac { 1 }{ { ((x+1)(x+2)(x+3)(x+4)) }^{ 3 } } \\ \\ \\ thus\quad we\quad can\quad see\quad that\quad it\quad would\quad form\quad a\quad telescopic\quad sum\\ where\quad all\quad the\quad terms\quad except\quad the\quad first\quad one\quad will\quad get\quad cancelled\quad out.\\ thus\quad our\quad answer\quad would\quad be\\ \frac { 1 }{ { (1*2*3*4) }^{ 3 } } *\frac { 1 }{ 4\\ } =\quad \frac { 1 }{ { (4!) }^{ 3 } } *\frac { 1 }{ 4 } \\ \\ thus\quad our\quad a\quad would\quad be\quad 4\quad and\quad b\quad would\quad be\quad 3.\\ thus\quad the\quad final\quad answer\quad would\quad be\quad B-A\quad =\quad 3-4\quad =\quad -1.\\ \\ \\ \\ \\ \\

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