Question

One sequence of alternating terms of
the series 1+2+3+4+5+8+... forms an
arithmetic progression, while the
other sequence of alternating terms
forms a geometric progression. Sum
the first 10 terms of each progression
and hence find the sum of the first 20
terms of the series.

Answer 1

clearly,
1,3,5 are an AP
2,4,8 are a GP

Now just plug in your formulas for partial sums.

Answer 2

I've found the first 10 terms of each progression, but m having problems with sum of the first 20 terms of the series. Am I going to solve for A.P and G.P differently.