## mathmath333 one year ago question permuttion

1. mathmath333

solve \large \color{black}{\begin{align} ^{5}P_{r}=^{6}P_{r-1}\hspace{.33em}\\~\\ \end{align}}

2. hartnn

so where are you stuck?

3. mathmath333

|dw:1439059294486:dw|

4. hartnn

|dw:1439059398107:dw|

5. hartnn

|dw:1439059437725:dw|

6. hartnn

i used the property that $$\huge n! = n(n-1)!$$

7. mathmath333

i m getting $$r=4,9$$

8. hartnn

good! :)

9. mathmath333

but the answer should be only single number

10. hartnn

but r can't be 4 why?

11. hartnn

i mean r can't be 9 :P

12. mathmath333

why???

13. hartnn

how would you calculate (5-9)! ?

14. mathmath333

-4!

15. mathmath333

$$\color{blue}{\text{Originally Posted by}}$$ @mathmath333 -4! $$\color{blue}{\text{End of Quote}}$$

16. mathmath333

hmm this extension works

17. hartnn

(-4)! factorial of a negative number isn't defined in real plane.

18. mathmath333

thanks

19. hartnn

welcome ^_^

20. mathmath333

$$\color{blue}{\text{Originally Posted by}}$$ @hartnn welcome ^_^ $$\color{blue}{\text{End of Quote}}$$

21. mathmath333

haha this funy quote

22. hartnn

$$\color{blue}{\text{Originally Posted by}}$$ @mathmath333 haha this funy quote $$\color{blue}{\text{End of Quote}}$$ indeed

23. mathmath333

$$\color{blue}{\text{Originally Posted by}}$$ @hartnn $$\color{blue}{\text{Originally Posted by}}$$ @mathmath333 haha this funy quote $$\color{blue}{\text{End of Quote}}$$ indeed $$\color{blue}{\text{End of Quote}}$$

24. hartnn

quoteception :P