If x is an integer. Is (X^2 +1) (X+5) an even number?
1. X is an odd no.
2. Each prime factor of X^2 is greater than 7.
GMAT prep prime factor
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Can you explain what statement #2 means? Thx.amitansu wrote:From 1 : x is odd
so, (x^2 +1) = even
and odd+odd = odd; finally, even X odd = even so, sufficient
From 2 :
x is always going to be an odd integer, so sufficient
Answer D.
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Each prime factor of x^2 is greater than 7 is the condition :
for example x^2=121 => x = 11
and 121= 11 x 11 (prime factor is 11 here which is greater than 7)
169=13 X 13 (13>7), so x is odd here.
likewise we have to test for different values of x whose each prime factors should be greater than 7.
Prime factors are those which are basically prime numbers as well as factors of x taking different values of it.
for example x^2=121 => x = 11
and 121= 11 x 11 (prime factor is 11 here which is greater than 7)
169=13 X 13 (13>7), so x is odd here.
likewise we have to test for different values of x whose each prime factors should be greater than 7.
Prime factors are those which are basically prime numbers as well as factors of x taking different values of it.
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Thanks!amitansu wrote:Each prime factor of x^2 is greater than 7 is the condition :
for example x^2=121 => x = 11
and 121= 11 x 11 (prime factor is 11 here which is greater than 7)
169=13 X 13 (13>7), so x is odd here.
likewise we have to test for different values of x whose each prime factors should be greater than 7.
Prime factors are those which are basically prime numbers as well as factors of x taking different values of it.
Any number can be broken down into powers of primes. The only even prime is 2. All primes of this number are greater than 7. Therefore, this number has all odd prime factors. Since odd * odd = odd, this number must therefore be odd. If this number is odd, we can determine that when plugged into the original equation, we always get an even number, so we can answer the stem definitively.
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Hey Maxx- Yes, 0 is indeed an even integer. I've been burned more than once by forgetting that.moneyman wrote:Great approach guys!!
Just one thing..what if x is -5 then it will be (26)(0)=0 so does this mean that 0 is an even integer??
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odd +odd = even and even * even = evenamitansu wrote:From 1 : x is odd
so, (x^2 +1) = even
and odd+odd = odd; finally, even X odd = even so, sufficient
From 2 :
x is always going to be an odd integer, so sufficient
Answer D.