Asked: If S is a sequence of 2 or more consecutive positive integers and if the sum of the terms in S is odd, which of the following must be true?
Quote:
1) The product of the terms in S is even
The product of the terms in S is even if any of the terms is even.
If the first term be odd, second term will be even since the terms are consecutive positive integers
MUST BE TRUE
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2) There is an odd number of terms in S
If the number of terms is 4, there are even number of terms
But if the number of terms is 3 or 5, there are odd number of terms
MAY BE TRUE
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3) The sum of the first and last terms in S is equal to the average (arithmetic mean) of the terms in S
Let first term be a.
and number of terms be n.
Last term l = a + (n-1) = a + n - 1
The sum of first and last terms in S = a + (a+n-1) = 2a + n - 1
Sum of all terms in S = n (2a + n -1)/2
Average of all terms in S = n (2a + n - 1)/2n = (2a + n - 1)/2
NEVER TRUE
(A) 1) only
(B) 1) and 2) only
(C) 1) and 3) only
(D) 2) and 3) only
(E) 1) 2), and 3
IMO A