.
The first two terms in a sequence are a and b. The third term in the sequence is the sum of the previous two terms.
The fourth term is the sum of the second and third terms. These sums give you the sequence a, b, a + b, a + 2b.
Using this pattern, which of the following expressions is the sixth term in the sequence?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1-st term: = a
2-nd term: = b
3-rd term: = a + b
4-th term: = b + (a+b) = a + 2b
5-th term: = = (a + b) + (a + 2b) = 2a + 3b.
6-th term: = = (a + 2b) + (2a + 3b) = 3a + 5b.
Solved.