Given that the sum of the first n terms of the sequence {an} is Sn, with a1=1 and an+1=1+2Sn. (1) Prove that the sequence {Sn} forms an arithmetic sequence. (2) Let cn=(Sn+1)2n, find the sum of the first n terms of the sequence {cn}, denoted as Tn.
Solution
### Solution:
#### Part (1): Proving that {Sn} forms an arithmetic sequence
Given that an+1=Sn+1−Sn and an+1=1+2Sn, we can equate these two expressions to get:
Sn+1−Sn=1+2Sn
Rearranging this equation, we find:
Sn+1=Sn+2Sn+1=(Sn+1)2
Taking the square root of both sides gives us:
Sn+1=Sn+1
This implies that the difference between consecutive terms of {Sn} is constant, equal to 1. Therefore, {Sn} forms an arithmetic sequence with the first term 1 and common difference 1.
#### Part (2): Finding the sum of the first n terms of the sequence {cn}
From part (1), we know that Sn=S1+(n−1)⋅1=n. Thus, we have:
cn=(n+1)2n
The sum of the first n terms of {cn}, denoted as Tn, can be written as:
Tn=2⋅2+3⋅22+4⋅23+⋯+(n+1)⋅2n(1)
Multiplying both sides of equation (1) by 2 gives:
2Tn=2⋅22+3⋅23+4⋅24+⋯+(n+1)⋅2n+1(2)
Subtracting equation (1) from equation (2) yields:
Tn=2⋅21+22+23+⋯+2n−(n+1)2n+1
Using the formula for the sum of a geometric series, we get:
Tn=2+1−22(1−2n)−(n+1)2n+1
Simplifying the expression, we find:
Tn=n⋅2n+1
Therefore, the sum of the first n terms of the sequence {cn} is Tn=n⋅2n+1. Encapsulating the final answer:
Tn=n⋅2n+1
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