Dấu :  \("."\)là dấu nhân nhé 

\(\frac{2}{1.3}+\frac{2}{3.5}+\frac{2}{5.7}+...+\frac{2}{77.79}\)

\(=1-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+...+\frac{1}{77}-\frac{1}{79}\)

\(=1-\frac{1}{79}\)

\(=\frac{79}{79}-\frac{1}{79}\)

\(=\frac{78}{79}\)

Chúc bạn học tốt !!! 

Giá trị biểu thức là 78/79

kiến thức lớp 8 chắc mới làm dc

\(A=\left(1+\frac{1}{\left(2-1\right)\left(2+1\right)}\right)\left(1+\frac{1}{\left(3-1\right)\left(3+1\right)}\right)+....+\frac{1}{\left(100-1\right)\left(100+1\right)}\)

\(A=\left(1+\frac{1}{2^2}\right)\left(1+\frac{1}{3^2}\right)......\left(1+\frac{1}{100^2}\right)\)

ok tự giải típ nhé

A=(1+1/1.3)+........+(1+1/99.100)

=>A=[ (1.3+1)/(1.3 ) ] .[ (2.4+1)/(2.4) ] .... [ (99.101+1)/(99.101) ] 

=>A=( 4/1.3 ).( 9/2.4)......( 10000/99.101)

=>A=( 2²/1.3).( 3²/2..4).......( 100²/99.101)

=>A=\(\frac{2^2.3^2........99^2.100^2}{1.3.2.4.....99.101}\)

=>A=\(\frac{2.3....100.2.3.....100}{1.2.....99.3.4.....101}\)

=>A=\(\frac{100.2}{101}\)

=>A=\(\frac{200}{101}\)

Vậy A=\(\frac{200}{101}\)

  − 26 .14 . − 5 : 70  =  − 26 . ( − 70 ) : 70 = − 26 . − 1 = 26

2/5 + 3/4 - 4/7 = 23/20 - 4/7 = 81/140

7/9 : 2/3 x 5/4 = 7/6 x 5/4 = 35/24

4/5 x (7/10 - 1/2) = 4/5 x 1/5 = 4/25

a) \(\dfrac{2}{5}+\dfrac{3}{4}-\dfrac{4}{7}\)

\(=\dfrac{56}{140}+\dfrac{105}{140}-\dfrac{80}{140}\)

\(=\dfrac{56+105-80}{140}\)

\(=\dfrac{81}{140}\)

b) \(\dfrac{7}{9}:\dfrac{2}{3}\times\dfrac{5}{4}\)

\(=\dfrac{7}{9}\times\dfrac{3}{2}\times\dfrac{5}{4}\)

\(=\dfrac{7\times3\times5}{9\times2\times4}\)

\(=\dfrac{35}{24}\)

c) \(\dfrac{4}{5}\times\left(\dfrac{7}{10}-\dfrac{1}{2}\right)\)

\(=\dfrac{4}{5}\times\left(\dfrac{7}{10}-\dfrac{5}{10}\right)\)

\(=\dfrac{4}{5}\times\dfrac{1}{5}\)

\(=\dfrac{4}{25}\)

\(2A=\frac{2}{1x3}+\frac{2}{3x5}+\frac{2}{5x7}+...+\frac{2}{999x1001}\)

\(2A=\frac{3-1}{1x3}+\frac{5-3}{3x5}+\frac{7-5}{5x7}+...+\frac{1001-999}{999x1001}\)

\(2A=\frac{3}{1x3}-\frac{1}{1x3}+\frac{5}{3x5}-\frac{3}{3x5}+\frac{7}{5x7}-\frac{5}{5x7}+...+\frac{1001}{999x1001}-\frac{999}{999x1001}\)

\(2A=1-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+...+\frac{1}{999}-\frac{1}{1001}\)

\(2A=1-\frac{1}{1001}=\frac{1000}{1001}\)=> A = 500/1001

\(\frac{500}{1001}\)!

\(A=3x^5-3x^4+5x^3-x^2+5x+2\)

\(\text{Thay x=-1 vào biểu thức A,ta được:}\)

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

\(A=3.\left(-1\right)-3.1+5.\left(-1\right)-1+5.\left(-1\right)+2\)

\(A=\left(-3\right)-3+\left(-5\right)-1+\left(-5\right)+2\)

\(A=\left(-6\right)+\left(-5\right)-1+\left(-5\right)+2\)

\(A=\left(-11\right)-1+\left(-5\right)+2\)

\(A=\left(-12\right)+\left(-5\right)+2\)

\(A=\left(-17\right)+2=-15\)

Thay x=-1 vào A ta có:
A= 3x⁵-3x⁴+5x³-x²+5x+2

   = 3.(-1)⁵-3.(-1)⁴+5.(-1)³-(-1)²+5.(-1)+2

    = 3.(-1)-3.1+5.(-1)-1+(-5)+2

    = -3-3-5-1-5+2

    =-15

ban vao cau hoi tuong tu nhe tic mjnh cai nhe

\(x=71\Leftrightarrow x-1=70\\ \Leftrightarrow A=x^5-\left(x-1\right)x^4-\left(x-1\right)x^3-\left(x-1\right)x^2-\left(x-1\right)x+34\\ A=x^5-x^5+x^4-x^4+x^3-x^3+x^2-x^2-x+x+34=34\)

\(A=x^5-70x^4-70x^3-70x^2-70x+34\)

\(=x^4\left(x-71\right)+x^3\left(x-71\right)+x^2\left(x-71\right)+x^2\left(x-71\right)+x\left(x-71\right)+x+34\)

\(=x^4\left(71-71\right)+...+x\left(71-71\right)+71+34\)

\(=x^4.0+...+x.0+105=105\)