Is 2^i + 1 a prime? 1) i^2 -4i +3= 0 2) 3^i + 1 is a prime

Question

Is 2^i + 1 a prime?

1) i^2 -4i +3= 0
2) 3^i + 1 is a prime

christoph · Answer

B)...

1) (i-1)(i-3)=0 => i=1 or i=3 => insuff

2) 3^0+1=2 => i=0 => suff

Vithal · Answer

thanks sparky - these little points got kind of blurred in my memory!

ywilfred · Answer

From (1), i=3 or i=1.
i=3 will give 2^i + 1=9 (not prime)
i=1 will give 2^i + 1=3 (prime)

So (1) is not sufficient

From (2), 3^i + 1 is a prime number.
3^i + 1 can be 2, i=0

For other values of i other than 0, 3^i + 1 is not a prime.

For i=0, 2^i + 1 is not prime. 

(2) is sufficient and therefore the answer is B

HowManyToGo · Answer

I too vote for B. Just wanted to hightlight the foll. In 2) 3^i + 1 is always even, why ? , 3^i is always odd and odd + odd = even. Hence the only prime which is even is 2 which we get when i=0. (am I getting brownie points here 8-)

) HMTG.

a4gmat · Answer

B

1)=> i=3 or i = 1 - insufficient
2)=> i=0 - sufficient

Is 2^i + 1 a prime? 1) i^2 -4i +3= 0 2) 3^i + 1 is a prime

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