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Algebra Difficulty 4.7 AIME Prove it

Given that the sum of the first nn terms of the sequence {an}\{a_{n}\} is Sn{S}_{n}, with a1=1{a}_{1}=1 and an+1=1+2Sn{a}_{n+1}=1+2\sqrt{{S}_{n}}.
(1)(1) Prove that the sequence {Sn}\{\sqrt{{S}_{n}}\} forms an arithmetic sequence.
(2)(2) Let cn=(Sn+1)2n{c}_{n}=(\sqrt{{S}_{n}}+1){2}^{n}, find the sum of the first nn terms of the sequence {cn}\{c_{n}\}, denoted as TnT_{n}.

Solution

### Solution:

#### Part (1): Proving that {Sn}\{\sqrt{S_{n}}\} forms an arithmetic sequence

Given that an+1=Sn+1Sna_{n+1} = S_{n+1} - S_{n} and an+1=1+2Sna_{n+1} = 1 + 2\sqrt{S_{n}}, we can equate these two expressions to get:

Sn+1Sn=1+2Sn S_{n+1} - S_{n} = 1 + 2\sqrt{S_{n}}

Rearranging this equation, we find:

Sn+1=Sn+2Sn+1=(Sn+1)2 S_{n+1} = S_{n} + 2\sqrt{S_{n}} + 1 = (\sqrt{S_{n}} + 1)^2

Taking the square root of both sides gives us:

Sn+1=Sn+1 \sqrt{S_{n+1}} = \sqrt{S_{n}} + 1

This implies that the difference between consecutive terms of {Sn}\{\sqrt{S_{n}}\} is constant, equal to 11. Therefore, {Sn}\{\sqrt{S_{n}}\} forms an arithmetic sequence with the first term 11 and common difference 11.

#### Part (2): Finding the sum of the first nn terms of the sequence {cn}\{c_{n}\}

From part (1), we know that Sn=S1+(n1)1=n\sqrt{S_{n}} = \sqrt{S_{1}} + (n-1) \cdot 1 = n. Thus, we have:

cn=(n+1)2n c_{n} = (n + 1)2^n

The sum of the first nn terms of {cn}\{c_{n}\}, denoted as TnT_{n}, can be written as:

Tn=22+322+423++(n+1)2n(1) T_{n} = 2 \cdot 2 + 3 \cdot 2^2 + 4 \cdot 2^3 + \cdots + (n + 1) \cdot 2^n \tag{1}

Multiplying both sides of equation (1) by 22 gives:

2Tn=222+323+424++(n+1)2n+1(2) 2T_{n} = 2 \cdot 2^2 + 3 \cdot 2^3 + 4 \cdot 2^4 + \cdots + (n + 1) \cdot 2^{n+1} \tag{2}

Subtracting equation (1) from equation (2) yields:

Tn=221+22+23++2n(n+1)2n+1 T_{n} = 2 \cdot 2^1 + 2^2 + 2^3 + \cdots + 2^n - (n + 1)2^{n+1}

Using the formula for the sum of a geometric series, we get:

Tn=2+2(12n)12(n+1)2n+1 T_{n} = 2 + \frac{2(1 - 2^n)}{1 - 2} - (n + 1)2^{n+1}

Simplifying the expression, we find:

Tn=n2n+1 T_{n} = n \cdot 2^{n+1}

Therefore, the sum of the first nn terms of the sequence {cn}\{c_{n}\} is Tn=n2n+1T_{n} = n \cdot 2^{n+1}. Encapsulating the final answer:

Tn=n2n+1 \boxed{T_{n} = n \cdot 2^{n+1}}

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.