\(=1+\frac{1}{3^1}+\frac{1}{3^2}+\frac{1}{3^3}+...+\frac{1}{3^7}\)

\(=1+\frac{1}{3}+\frac{1}{9}+\frac{1}{27}+\frac{1}{81}+\frac{1}{243}+\frac{1}{729}+\frac{1}{2187}\)

\(=\frac{2187}{2187}+\frac{729}{2187}+\frac{81}{2187}+\frac{27}{2187}+\frac{9}{2187}+\frac{3}{2187}+\frac{1}{2187}\)

\(=\frac{2187+729+81+27+9+3+1}{2187}\)

\(=\frac{3037}{2187}\)

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toán lớp 4 chưa học bình phương -.-

( x - 1 )²⁰¹⁸ + (y - 2 )²⁰²⁰+(z-3)²⁰²²=0

\(\Leftrightarrow\left\{{}\begin{matrix}x-1=0\\y-2=0\\z-3=0\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}x=1\\y=2\\z=3\end{matrix}\right.\)

\(A=\dfrac{1}{9}\left(-x\right)^{2021}y^2z^3=\dfrac{1}{3}\left(-1\right)^{2021}.2^2.3^3=\dfrac{1}{3}.\left(-1\right).4.27=-36\)

câu 1: =15

câu 2:=-98

câu 3:   54-(-16)-(-13)+27

         =     70   -    14

         =           56

\(\frac{1.5.18+2.10.36+3.15.54}{1.3.9+2.6.18+3.9.27}\)

\(=\frac{90+720+2340}{27+216+729}\)

\(=\frac{3150}{972}=\frac{175}{54}\)

Lời giải:

Đặt $A=1+\frac{1}{3}+\frac{1}{9}+\frac{1}{27}+...+\frac{1}{2187}$
$3\times A=3+1+\frac{1}{3}+\frac{1}{9}+...+\frac{1}{729}$

$3\times A-A=3-\frac{1}{2187}$

$2\times A=3-\frac{1}{2187}=\frac{6560}{2187}$

$A=\frac{6560}{2187}:2=\frac{3280}{2187}$

1/ Đặt Biểu thức cần tính là A

\(A=\frac{1}{2}+\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+\frac{1}{30}+\frac{1}{42}=\frac{1}{1x2}+\frac{1}{2x3}+\frac{1}{3x4}+\frac{1}{4x5}+\frac{1}{5x6}+\frac{1}{6x7}\)

\(A=\frac{2-1}{1x2}+\frac{3-2}{2x3}+\frac{4-3}{3x4}+\frac{5-4}{4x5}+\frac{6-5}{5x6}+\frac{7-6}{6x7}\)

\(A=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}=1-\frac{1}{7}=\frac{6}{7}\)

2/

Phân số chỉ lượng nước 2 vòi cùng chảy trong 1 giờ là

1:4=1/4 bể

Phân số chỉ lượng nước vòi 1 chảy trong 1 giờ là

1:5=1/5 bể

Phân số chỉ lượng nước vòi 2 chảy trong 1 giờ là

1/4-1/5=1/20 bể

Thời gian vòi hai chảy một mình đầy bể là

1:1/20=20 giờ

3/

\(3xA=1+\frac{1}{3}+\frac{1}{9}+\frac{1}{27}+\frac{1}{81}\)

\(2xA=3xA-A=1-\frac{1}{243}=\frac{242}{243}\Rightarrow A=\frac{121}{243}\)

4/ x=2

x = 2

Đúng 100% luôn . Ok meo nhé . hi - hi- hi . ha - ha - ha . ngốc thế .

Ta có:\(A=\frac{1}{3}+\frac{1}{9}+\frac{1}{27}+\frac{1}{81}+\frac{1}{243}\)

Xét\(\frac{1}{3}A=\frac{1}{9}+\frac{1}{27}+\frac{1}{81}+\frac{1}{243}+\frac{1}{729}\)

\(\Leftrightarrow A-\frac{1}{3}A=\frac{1}{3}-\frac{1}{729}\)

\(\Leftrightarrow\frac{2}{3}A=\frac{243-1}{729}\Leftrightarrow A=\frac{3}{2}\times\frac{242}{729}=\frac{121}{243}\)

Phải là : A=1/3+1/9+1/27+1/81+1/243 ta có: 3A=1+1/3+1/9+1/27+1/81                     3A-A=(1+1/3+1/9+1/27+1/81)-(1/3+1/9+1/27+1/81+1/243)=1-1/243       2A=242/243                                             A=242/243:2=121/243

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

Ax3=(1/3 + 1/9 + 1/27 + 1/81 + 1/243)x3

Ax3=1/3 x 3 + 1/9 x 3 + 1/27 x 3  + 1/81 x 3 + 1/243 x 3

Ax3=1+1/3+1/9+1/27+1/81

Ax3-A=(1+1/3+1/9+1/27+1/81)-(1/3+1/9+1/27+1/81+1/243)

Ax(3-1)=1-1/243

Ax2=243/243-1/243

Ax2=      242/243

A   = 242/243:2

A    =  242/243 x 1/2

A    =       121/243

Vậy A= 121/243

Li-ke cho mik nhé!

ai lm tr mình cho tích