Ta có:

\(A=3+3^2+3^3+...+3^{2009}\)

\(3A=3^2+3^3+3^4+...+3^{2010}\)

\(3A-A=2A=\left(3^2+3^3+3^4+...+3^{2010}\right)-\left(3+3^2+3^3+...+3^{2009}\right)\)

\(2A=3^{2010}-3\)

  \(A=\frac{3^{2010}-3}{2}\)

Ta có:

2A + 3 = 3²⁰¹⁰ - 3 + 3 = 3²⁰¹⁰ 

=> n = 2010

Vậy n = 2010

ỦNG HỘ NHA

BẰNG 602 

K NHA

\(10+10+20+20+30+30=120\)

\(\frac{4}{5}+\frac{5}{5}+\frac{6}{5}+\frac{7}{5}+5=\frac{4+5+6+7}{5}=\frac{22}{5}+\frac{5}{1}=\frac{22}{5}+\frac{25}{5}=\frac{22+25}{5}=\frac{47}{5}\)

K mình nha    Ami Mizuno

Mọi người k mình nữa nha!

126 - 126 . (x - 6) = 99

126 . [ 1 - ( x - 6)] = 99

126 . [ 1 - x + 6 ] = 99

126 . [ 7 - x ] = 99

7 - x = 11/14

x = 7 - 11/14 = 87/14

Ta có :

\(\frac{7}{6}=\frac{7\cdot8}{6\cdot8}=\frac{56}{48}\)  \(\frac{9}{8}=\frac{9\cdot6}{8\cdot6}=\frac{54}{58}\)

Vậy : \(\frac{7}{6}>\frac{9}{8}\)

\(125-125.\left(x-5\right)=100\)

\(125.\left(x-5\right)=125-100\)

\(125.\left(x-5\right)=25\)

\(x-5=25:125\)

\(x-5=0,2\)

\(x=0,2+5\)

\(x=5,2\)

K mình nha    Girl xinh béo mập

Mình nhanh nhất đó!

Ta so sánh:\(\frac{23}{88}và\frac{13}{38}\)

Ta có:\(\frac{23}{88}< \frac{23}{69}=\frac{1}{3}=\frac{13}{39}< \frac{13}{38}\)

\(\Rightarrow\frac{23}{88}< \frac{13}{38}\Rightarrow-\frac{23}{88}>-\frac{3}{38}\)