Đặt \(A=\frac{1}{3}+\frac{1}{9}+\frac{1}{27}+\frac{1}{81}+\frac{1}{243}\)

\(A=\frac{1}{3}+\frac{1}{3^2}+\frac{1}{3^3}+\frac{1}{3^4}+\frac{1}{3^5}\)

\(3A=1+\frac{1}{3}+\frac{1}{3^2}+\frac{1}{3^3}+\frac{1}{3^4}\)

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

\(2A=1-\frac{1}{3^5}\)

\(A=\frac{242}{243}:2\)

\(A=\frac{121}{243}\)

Ủng hộ mk nha !!! ^_^

khong sai de bai dau tran quoc viet a

x + 80 : 2 = 9 + 65

x + 40 = 74

x = 74 - 40

x = 34

Yêu cầu dùng máy tính đi bạn . Ngồi mà bấm chắc gãy hết cả tay .

a, 35x 34+35x 38 + 65x 75+ 65x 45=10437

b, 36x 28 + 36x 82 + 64x 69 + 64 x41=11000

c, 254x 399 - 145/ 254+ 399 x 253=202292,4291

=81/243+27/243+9/243+3/243+1/243

=121/243

\(\left(1-\frac{1}{3}\right).\left(1-\frac{1}{6}\right).\left(1-\frac{1}{10}\right)...\left(1-\frac{1}{190}\right)\)

\(=\frac{2}{3}.\frac{5}{6}.\frac{9}{10}....\frac{189}{190}\)

\(=\frac{4}{6}.\frac{10}{12}.\frac{18}{20}....\frac{378}{380}\)

\(=\frac{1.4}{2.3}.\frac{2.5}{3.4}.\frac{3.6}{4.5}....\frac{18.21}{19.20}\)

\(=\frac{1.2.3....18}{2.3.4....19}.\frac{4.5.6...21}{3.4.5....20}\)

\(=\frac{1}{19}.\frac{21}{3}\)

\(=\frac{1}{19}.7=\frac{7}{19}\)

Ủng hộ mk nha !!! ^_^

Đua xe đạp bạn ạ.

a) |x-1|+|y+1|=0

Vì \(\hept{\begin{cases}\left|x-1\right|\ge0\\\left|y+1\right|\ge0\end{cases}\Rightarrow\left|x-1\right|+\left|y+1\right|\ge0\Rightarrow x-1+y+1=0\Rightarrow x+y=0}\)

Vậy x và y là số đối của nhau thỏa mãn đề bài

Có, giúp mk với !