PS

This topic has expert replies
Master | Next Rank: 500 Posts
Posts: 128
Joined: Sat Nov 17, 2007 11:59 am
Thanked: 1 times

PS

by cris » Wed Jan 30, 2008 9:33 am
For every positive even integer N, the function H(N) is defined to be the product of all the even integers from 2 to N, inclusive. If P is the smallest prime factor of H(100)+1, then P is ?

OA: [spoiler]>40[/spoiler]
Source: — Problem Solving |

User avatar
Legendary Member
Posts: 986
Joined: Wed Dec 20, 2006 11:07 am
Location: India
Thanked: 51 times
Followed by:1 members

by gabriel » Wed Jan 30, 2008 10:09 am
According the given information h(100)= 2*4*6*8*10......*100

If we take 2 as common from each of those even number we get h(100)= 2^50 (1*2*3*4*.....*50).

Now, Consider any number, eg. 30 = 1*2*3*5 the prime factors over here are 2,3,5. But if we add 1 to 30 the new number (in this case 31) will not be divisible by any of 30's prime factor i.e. 2,3,5 because though 30 is divisible by 2,3,5 .. 1 is not divisible by either of the three prime factors 2,3,5.

For h(100) the prime factors are all the prime numbers between 1 and 50. Now, when we add 1 to h(100) the resulting number will not be divisible by any of the prime factors of h(100) (the same reason as in the above example)and the smallest prime factor will have to be greater than 49.

Master | Next Rank: 500 Posts
Posts: 128
Joined: Sat Nov 17, 2007 11:59 am
Thanked: 1 times

by cris » Wed Jan 30, 2008 10:30 am
Thanks a lot gabriel...but why it has to be >49 and not >50?

Update
---------------------

Yhinking abiut it it doesnt really matter bc neither 49 nor 50 are primes right? :D

thnaks again gabriel!