chetan2u
If x = n!+1, is x a prime number?
(1) x has no factor other than 1 common with any number less than or equal to n.
(2) x has no factor other than 1 common with any integer between n and n! , inclusive.
self made
OA in 2 days
x = n!+1...
Now n! is a multiple of all integers till n, so n!+1 will be co-prime to each integer till n..
BUT we require to know if x has any common factor other than 1 with integers from n to n!lets see the statements
(1) x has no factor other than 1 common with any number less than or equal to n.No NEW info..
as can be seen x is co-prime with all integers till n, inclusive
so insuff
(2) x has no factor other than 1 common with any integer between n and n! , inclusive.this says that all integers between n and n!, inclusive are co-prime to x..
we already know as x=n!+1 that x is co-prime to all integers till n, inclusive..
so x does not have any integer less than itself that has any other factor except 1 common with x..
Suff
B
example-
3! + 1 = 7.. satisfies the statement II and thus is PRIME..
On other hand -
\(4! + 1 =25..\)
now x is co-prime with all integers till 4, BUT not from 4 to 24, as it has common factors with 5,10,15,and 20.. hence does not follow statement II and is not a PRIME..