Polynomials And Factorials Again

n ! = 5 × 5 × 5 5 \large n! = 5\times5\times5 - 5

What is the value of n n ?

4 5 6 7

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4 solutions

Achille 'Gilles'
Mar 20, 2016

5x5x5-5=120

120÷2=60

60÷2=30

30÷2=15

15÷3=5

5÷5=1

1x2x3x(2x2)x5=5!

Jason Li
Mar 20, 2016

5x5x5-5 reduces to: 5(5^2-1) Factoring gives 5(6)(4) Since prime factorization of 6 is 2x3, then this becomes 2x3x4x5, which is 5!.

Moderator note:

Simple standard approach.

By BODMAS, n ! = 120 n!=120 or n = 5 n=5 .

This is a factoral method:

n ! = 5 5 5 5 n ! = 125 5 n ! = 120 \displaystyle{\begin{aligned} n! =5 \cdot 5 \cdot 5 - 5 &\Rightarrow n! = 125 - 5 \\&\Rightarrow n! = 120 \end{aligned}} .

You don't have to think a number for factoral to find the value of n n you have to find form 0 to 9 to find the equal figure.

We have to try:

For 2 2 : let n = 2 n = 2 So

n ! = 120 2 ! = 120 2 = 120 2 ! = 1 2 2 2 120 \begin{aligned}n!=120 &\Rightarrow 2!=120 \\&\Rightarrow 2 = 120 \quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad{ 2! =1 \cdot 2 \Rightarrow 2 } \\&\Rightarrow 2 \not = 120 \end{aligned}

For 4 4 : let n = 2 n = 2 So

n ! = 120 4 ! = 120 4 ! = 1 2 3 4 = 24 24 120 \begin{aligned} n! =120 &\Rightarrow 4! = 120 \quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad{4! = 1 \cdot 2 \cdot 3 \cdot 4 \Rightarrow = 24 }\\&\Rightarrow 24 \not = 120 \end{aligned}

For 5 5 : let n = 5 n = 5 So

n ! = 120 5 ! = 120 5 ! = 1 2 3 4 5 120 120 = 120 \begin{aligned} n! = 120&\Rightarrow 5! =120 \quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad{5! = 1 \cdot 2 \cdot 3 \cdot 4 \cdot 5 \Rightarrow 120} \\&\Rightarrow 120 = 120 \end{aligned}

\therefore We found the value of the n n that makes the 120 = 5 ! 120 = 5! . So the n = 5 n = 5

FIN!!!! \large \text{FIN!!!!}

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