Factoring Factorial Part 1

Let's rewrite 10 ! 10! as 3 a × 1 0 b × 14 c × 32 d 3^{ a }\quad \times \quad 10^{ b }\quad \times \quad { 14 }^{ c }\quad \times \quad { 32 }^{ d } . Which of these could be the answer to d a b c d\sqrt { abc }

6 2 2 2\sqrt { 2 } 2 1 0 2 \sqrt { 2 } 6 \sqrt { 6 }

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1 solution

Abyoso Hapsoro
May 4, 2015

First off, let's rewrite 10 ! 10! with its prime factors. 1) Calculate 2 A { 2 }^{ A } A = 1 10 2 A = 10 2 + 10 4 + 10 8 + . . . = 5 + 2 + 1 + . . . = 8 \sum _{ A=1 }^{ \infty }{ \left\lfloor \frac { 10 }{ { 2 }^{ A } } \right\rfloor } \quad =\quad \left\lfloor \frac { 10 }{ 2 } \right\rfloor \quad +\quad \left\lfloor \frac { 10 }{ 4 } \right\rfloor \quad +\quad \left\lfloor \frac { 10 }{ 8 } \right\rfloor \quad +\quad ...\quad =\quad 5\quad +\quad 2\quad +\quad 1\quad +\quad ...\quad =\quad 8 2) Calculate 3 B { 3 }^{ B } B = 1 10 3 B = 10 3 + 10 9 + . . . = 3 + 1 + . . . = 4 \sum _{ B=1 }^{ \infty }{ \left\lfloor \frac { 10 }{ { 3 }^{ B } } \right\rfloor } \quad =\quad \left\lfloor \frac { 10 }{ 3 } \right\rfloor \quad +\quad \left\lfloor \frac { 10 }{ 9 } \right\rfloor \quad +\quad ...\quad =\quad 3\quad +\quad 1\quad +\quad ...\quad =\quad 4 3) Calculate 5 C { 5 }^{ C } C = 1 10 5 C = 10 5 + . . . = 2 + . . . = 2 \sum _{ C=1 }^{ \infty }{ \left\lfloor \frac { 10 }{ { 5 }^{ C } } \right\rfloor } \quad =\quad \left\lfloor \frac { 10 }{ 5 } \right\rfloor \quad +\quad ...\quad =\quad 2\quad +\quad ...\quad =\quad 2 4) Calculate 7 D { 7 }^{ D } D = 1 10 7 D = 10 7 + . . . = 1 + . . . = 1 \sum _{ D=1 }^{ \infty }{ \left\lfloor \frac { 10 }{ { 7 }^{ D } } \right\rfloor } \quad =\quad \left\lfloor \frac { 10 }{ 7 } \right\rfloor \quad +\quad ...\quad =\quad 1\quad +\quad ...\quad =\quad 1 Therefore, 10 ! = 2 A × 3 B × 5 C × 7 D = 2 8 × 3 4 × 5 2 × 7 1 10!\quad =\quad { 2 }^{ A }\quad \times \quad { 3 }^{ B }\quad \times \quad { 5 }^{ C }\quad \times \quad { 7 }^{ D }\quad =\quad { 2 }^{ 8 }\quad \times \quad { 3 }^{ 4 }\quad \times \quad { 5 }^{ 2 }\quad \times \quad { 7 }^{ 1 } Now, let's rewrite 10 ! 10! as what the question asks for. 3 will stay as 3, there fore a = 4 a = 4 , 2 and 5 will create 10 since 5 does not go into another exponent, therefore b = 2 b = 2 , 2 and 7 will create 14 since 7 does not go into another exponent, therefore c = 1 c = 1 , the rest is 2 5 { 2 }^{ 5 } or 32, therefore d = 1 d = 1 . d a b c = 1 4 × 2 × 1 = 8 = 2 2 d\sqrt { abc } \quad =\quad 1\sqrt { 4\times 2\times 1 } \quad =\quad \sqrt { 8 } \quad =\quad 2\sqrt { 2 }

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