B=2.22+3.23+4.24+......+10.210

Hãy so sánh B với 214

Nhanh nhất, cụ thể và đúng nhất, 10k

A=1+2+2^2+2^3+...+2^10

=>2A=2+2^2+2^3+2^4+...+2^11

=>2A-A=2+2^2+2^3+2^4+...+2^11-(1+2+2^2+2^3+...+2^10)

=>A=1+2^11>2^11

_{\(A=1+2+2^2+2^3+...+2^{10}\)}

_{\(2A=2+2^2+2^3+2^4+...+2^{11}\)}

^{\(2A-A=\left(2+2^2+2^3+2^4+...+2^{11}\right)-\left(1+2+2^2+2^3+...+2^{10}\right)\)}

^{\(A=2^{11}-1< 2^{11}\)}

^{\(B=2.2^2+3.2^3+4.2^4+...+10.2^{10}\)\(2B=2.2^3+3.2^4+4.2^5+...+10.2^{11}\)\(2B-B=\left(2.2^3-3.2^3\right)+\left(3.2^4-4.2^4\right)+...+\left(9.2^{10}-10.2^{10}\right)+10.2^{11}-2.2^2\)\(B=2^3\left(2-3\right)+2^4\left(3-4\right)+...+2^{10}\left(9-10\right)+10.2^{11}-2.2^2\)\(B=-2^3-2^4-....-2^{10}+10.2^{11}-2^3\)}

_{\(B=-\left(2^3+2^4+...+2^{10}\right)+10.2^{11}-2^3\)}

^{\(B=-\left(2^{11}-2^3\right)+10.2^{11}-2^3\)}

_{\(B=-2^{11}+2^3+10.2^{11}-2^3\)}

_{\(B=9.2^{11}\)}

Ta cần so sánh: _\(9.2^{11}\) và _\(2^{14}\)

Hay _\(9\) và _\(2^3\)

_{\(9>8=2^3\Leftrightarrow B>2^{14}\)}

Bài làm 

Đặt a - b = x ; b - c = y ; c - a = z 

 => x + y + z = 0

 Ta có :

          \(N=\left(\frac{1}{x}+\frac{1}{y}+\frac{1}{z}\right)^2-2.\left(\frac{1}{xy}+\frac{1}{yz}+\frac{1}{xz}\right)=\left(\frac{1}{x}+\frac{1}{y}+\frac{1}{z}\right)^2-2.\left(\frac{x+y+z}{xyz}\right)\)

=>     \(N=\left(\frac{1}{x}+\frac{1}{y}+\frac{1}{z}\right)^2\)( Vì x + y + z = 0 )

Vậy ta có đpcm