GMAT CD...HELP

Question 1 (in future, please only post 1 question per thread, makes it easier for everyone to follow along):

First, we need to understand the sequence.

Basically, odd numbered terms equal the number of the term. So, the 1st term =1, the 3rd term =3, the 5th term =5 and so on.

Even numbered terms equal negative the previous term. So, the 2nd term = -1, the 4th term = -3, the 6th term = -5 and so on.

So, the sequence is:

{1, -1, 3, -3, 5, -5, 7, -7, ...}

The question is:

Is the sum of the terms positive?

Looking at our sequence, we can see that each pair of terms sums to 0. So, if we have an odd number of terms, the sum will be positive; if we have an even number of terms, the sum will be 0.

Let's look at the statements:

(1) n is odd.

Odd number of terms, therefore YES, the sum will be positive: sufficient.

(2) a(n) is positive.

If the nth term is positive, then n must be an odd number (since all the even numbered terms are negative). Therefore we have an odd number of terms and YES, the sum will be positive: sufficient.

Each of 1 and 2 is sufficient alone: choose (d).

Question 2:
Here we do not have knowledge about the series. Neither we can deduce any thing from choices.
A: As we have no knowledge of series we cannot deduce n by formula. So A us ruled out.
B: Average is calculated by adding all numbers and division by n. But here sum of all numbers is unknown. So B is ruled out.
C: 1 gives us sum of numbers and 2 gives us average.
n = 3124/4 = 781

(A)

Stmt I.

xyz =70. If you breakup 70 to prime factors, result is 2x5x7. This is it.
x+y+z = 2+5+7 = 14.

Stmt II.

10x = 7yz (this is valid for many different values of x, y and z). Not suff.

Ans. A

Question1)

From the sequence we get to know it will be of the form

1,-1,3,-3,5,5,,......

A) n=1,3,5
if n=1 , sum=+
if n=3 sum +
if we observe it cancels out the rest excpet the last term which is always psotive

B)an=positive only when N is odd

IMO D


Question-2

t1,t2,t3,...tn

A) sum is 3124 !!

but is it GP or AP we dont know Insuff

B again average of is 4 !! but n not known Isuffect

Together

Avg=Sum/n

4=3124/n



C is IMO


question 3)

x>1,y>1,z>1

x+y+z=????


A)xyz=70

nothing no info !! Insuff

B)we have the ration not the values !! Insusff

together,

we can find out x=7 but yx is 10 !! can be any value between 1-10 !!

Not suff

IMO E