St 1)
Let's denote the first two terms as x and y.
According to the given information, the third term is calculated as ((x + y)/2),
The fourth term is then ((y + (x + y)/2)/2).
Now, since the value of x and y can change, so the sum of first four will also change.
Not Suff.St2)
Let's denote the first three terms as x, y and z
Sum of x, y and z = 30
Since there is no info about the fourth term, the sum of the first 4 terms cannot be found.
Not Suff.Let's combine St1 and St2
Considering first 2 terms as x and y , we can form the following equation:
x + y + x/2 + y/2 = 30
or, x + y = 20
this will only gives that the third term is 20/2 = 10. and the sum of first 2 term is 20. It is not possible to find the 4th term unless we know the value of 2nd term as fourth term is the avg of 2nd and 3rd term .
Hence, even after combining, it is NOT possible to find the sum of first 4 terms.
Not Suff
Answer EAbdurRakib
What is the sum of the first four terms of sequence S ?
(1) After the first two terms of S, the value of each term of S is equal to the average (arithmetic mean) of the last two preceding terms.
(2) The average (arithmetic mean) of the first three terms of S is 10.
OG Q 2017 New Question (Book Question: 219)