binominal (1 Viewer)

shuning · Jul 9, 2009

find n
$n\binom{n-1}{1}+n\binom{n-1}{2}+.....+n\binom{n-1}{n-2}>20 000$

My working:
$n\binom{n-1}{1}+n\binom{n-1}{2}+.....+n\binom{n-1}{n-2}$
$=n ((1+1)^{n-1}-2)$

$n ((1+1)^{n-1}-2) > 20 000$

what do i do now? or did i get wrong?

lyounamu · Jul 9, 2009

shuning said:
find n
$n\binom{n-1}{1}+n\binom{n-1}{2}+.....+n\binom{n-1}{n-2}>20 000$

My working:
$n\binom{n-1}{1}+n\binom{n-1}{2}+.....+n\binom{n-1}{n-2}$
$=n ((1+1)^{n-1}-2)$

$n ((1+1)^{n-1}-2) > 20 000$

what do i do now? or did i get wrong?

u are right.

just guess and check i guess xD.
n>=12

shuning · Jul 9, 2009

回复: Re: binominal

no way i can do it w/o guessing?

lyounamu · Jul 9, 2009

Re: 回复: Re: binominal

shuning said:
no way i can do it w/o guessing?

guess and check is pretty fast u know xD.

binominal (1 Viewer)

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