## virtus 3 years ago question below

1. virtus

\[nC4 + nC5\]

2. akash123

use d formula for n c r and do d simplification

3. akash123

nCr= n!/ r! (n-r)!

4. virtus

is it ok if you show me, kinda new to this

5. akash123

n!/ 4! (n-4)! + n!/ 5! (n-5)!

6. akash123

n(n-1)(n-2)(n-3)/24 + n(n-1)(n-2)(n-3)(n-4)/ 120

7. akash123

it's ok?

8. virtus

i still can't see it

9. akash123

see... n!= 1.2.3.4....n

10. akash123

right?

11. virtus

yes

12. akash123

n!/ (n-4)! = 1.2.3....(n-4)(n-3)(n-2)(n-1)n / (1.2.3....(n-4))

13. virtus

yeah...

14. akash123

so now u can solve ur problem....:)

15. virtus

let me try

16. virtus

n(n-1)(n-2)(n-3)/24 + n(n-1)(n-2)(n-3)(n-4)/ 120

17. virtus

i got that, but how do you simplify further?

18. virtus

LOL @hba looks like you have no choice but to help this damsel in distress

19. hba

@virtus i dont think so lol

20. virtus

16Cn +16C9 =17C9

21. virtus

FIND "n"

22. virtus

why not @hba

23. hba

Its been along time ive studied it and i dont want to get you into another confusion :(

24. virtus

OH LOL ok then

25. akash123

16Cn +16C9 =17C9 16Cn+16C9=16C9 + 16C8 16Cn=16C8 n=8

26. akash123

use the identity nCr + nC(r-1) = (n+1)Cr