Đặt \(A=2.2^2+3.2^3+4.2^4+5.2^5+...+n.2^n\)

\(\Rightarrow2A=2.2^3+3.2^4+4.2^5+5.2^6+...+n.2^{n+1}\)

\(\Rightarrow2A-A=2.2^3+3.2^4+4.2^5+5.2^6+...+n.2^{n+1}\)

\(-2.2^2-3.2^3-4.2^4-5.2^5-...-n.2^n\)

\(A=n.2^{n+1}-2^3-\left(2^3+2^4+...+2^n\right)\)

Đặt \(M=\left(2^3+2^4+...+2^n\right)\)

\(\Rightarrow2M=\left(2^4+2^5+...+2^{n+1}\right)\)

\(\Rightarrow M=2^{n+1}-2^3\)

\(\Rightarrow A=n.2^{n+1}-2^3-2^{n+1}+2^3\)

\(\Rightarrow A=\left(n-1\right)2^{n+1}=2^{n+10}\)

\(\Rightarrow\left(n-1\right)=2^9\)

\(\Rightarrow n=513\)

Ta có: \(2\cdot2^2+3\cdot2^2+...+n\cdot2^2=2^{n+10}\)

\(\Leftrightarrow2^2\cdot\left(2+3+...+n\right)=2^{n+10}\)

\(\Leftrightarrow\frac{\left(n+2\right)\left[\left(n-2\right)\div1+1\right]}{2}=2^{n+8}\)

\(\Leftrightarrow\left(n+2\right)\left(n+1\right)=2^{n+9}\)

Mà trong n+1 và n+2 luôn tồn tại 1 số lẻ và 2ⁿ⁺⁹ là lũy thừa của 2 nên ta xét 2 TH sau:

Nếu \(n+1=1\Rightarrow n=0\) thử lại ta thấy không thỏa mãn

Nếu \(n+2=1\Rightarrow n=-1\left(ktm\right)\) vì n là STN

Vậy không tồn tại số n thỏa mãn

Đặt \(A=2.2^2+3.2^3+4.2^4+...+n.2^n=2^{n+10}\)

\(\Rightarrow2A=2.2^3+3.2^4+4.2^5+...+n.2^{n+1}\)

\(\Rightarrow2A-A=2.2^3+3.2^4+4.2^5+...+n.2^{n+1}-2.2^2-3.2^3-4.2^4-...-n.2^n\)

\(\Leftrightarrow A=-2.2^2+\left(2.2^3-3.2^3\right)+\left(3.2^4-4.2^4\right)+...+[\left(n-1\right)2^n-n.2^n]+n.2^{n+1}\)

\(\Leftrightarrow A=-2.2^2-2^3-2^4-...-2^n+n.2^{n+1}\)

\(\Leftrightarrow A=-2^3-\left(2^4-2^3\right)-\left(2^5-2^4\right)-...-\left(2^{n+1}-2^n\right)+n.2^{n+1}\)

\(\Leftrightarrow A=-2^3-2^4+2^3-2^5+2^4-...-2^{n+1}+2^n+n.2^{n+1}\)

\(\Leftrightarrow A=-2^{n+1}+n.2^{n+1}\)

\(\Leftrightarrow A=2^{n+1}\left(n-1\right)\)

Mà \(A=2^{n+10}=2^{n+1}.2^9=2^{n+1}.512\)

\(\Rightarrow n-1=512\)

\(\Rightarrow n=513\)

2ⁿ⁺² - 3 . 2^n-1 = 5 . 2⁴

2ⁿ⁺² - 3 . 2^n-1 = 5 . 16

2ⁿ⁺² - 3 . 2^n-1 = 80

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