1,2 x 5,5 x 3+ 1,8 x 4,5 x 2 

= (1,2 x 3 ) x 5,5 + (1,8 x 2) x 4,5 

= 3,6 x 5,5 + 3,6 x 4,5

= 3,6 x (5,5+4,5)

= 3,6 x 10

= 36

= 36

\(1,2\times5,5\times3+1,8\times4,5\times2\\ =\left(1,2\times3\right)\times5,5+\left(1,8\times2\right)\times4,5\\ =3,6\times5,5+3,6\times4,5\\ =3,6\times\left(5,5+4,5\right)\\ =3,6\times10\\ =36\)

\(\dfrac{1}{1.2}+\dfrac{1}{2.3}+\dfrac{1}{3.4}+\dfrac{1}{4.5}+...+\dfrac{1}{x.\left(x+1\right)}=\dfrac{499}{500}\)

\(\Leftrightarrow1-\dfrac{1}{2}+\dfrac{1}{2}-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{4}+...+\dfrac{1}{x}-\dfrac{1}{x+1}=\dfrac{499}{500}\)

\(\Leftrightarrow1-\dfrac{1}{x+1}=\dfrac{499}{500}\)

\(\Leftrightarrow x=499\)

giúp e vs

     \(\frac{1313}{1212}:x=\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+\)\(\frac{1}{5.6}\)

\(\Leftrightarrow\frac{13}{12}:x=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}\)

\(\Leftrightarrow\frac{13}{12}:x=1-\frac{1}{6}\)

\(\Leftrightarrow\frac{13}{12}:x=\frac{5}{6}\)

\(\Leftrightarrow x=\frac{13}{12}:\frac{5}{6}\)

\(\Leftrightarrow x=\frac{13}{10}\)

  Vậy \(x=\frac{13}{10}\)

                            ~~~~~Hok tốt ~~~~~

a,\(\frac{1313}{1212}\div x=\frac{1}{1\times2}+\frac{1}{2\times3}+\frac{1}{3\times4}+\frac{1}{4\times5}+\frac{1}{5\times6}\)

\(\frac{13}{12}\div x=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}\)

\(\frac{13}{12}\div x=1-\frac{1}{6}\)

\(\frac{13}{12}\div x=\frac{5}{6}\)

\(x=\frac{13}{12}\div\frac{5}{6}\)

\(x=\frac{13}{12}\times\frac{6}{5}\)

\(x=\frac{13}{10}\)

Chúc bạn hok tốt !

\(\frac{1313}{1212}:x=\frac{1}{1x2}+\frac{1}{2x3}+...+\frac{1}{5x6}\)

\(\frac{1313}{1212}:\frac{101}{101}=\frac{13}{12}:x=\frac{1}{1}-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+...+\frac{1}{5}-\frac{1}{6}\)

\(\frac{13}{12}:x=1-\frac{1}{6}\)

\(\frac{13}{12}:x=\frac{5}{6}\)

\(x=\frac{13}{12}:\frac{5}{6}\)

\(x=\frac{78}{60}=\frac{13}{10}\)

       \(\frac{1313}{1212}:x=\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+\)\(\frac{1}{5.6}\)

\(\Leftrightarrow\frac{13}{12}:x=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}\)

\(\Leftrightarrow\frac{13}{12}:x=1-\frac{1}{6}\)

\(\Leftrightarrow\frac{13}{12}:x=\frac{5}{6}\)

\(\Leftrightarrow x=\frac{13}{12}:\frac{5}{6}\)

\(\Leftrightarrow x=\frac{13}{10}\)

                  Hok tốt

khó vậy

\(x-\frac{1}{1.2}-\frac{1}{2.3}-\frac{1}{3.4}=\frac{1}{4.5}\)

\(x-\frac{1}{1.2}-\frac{1}{2.3}-\frac{1}{3.4}-\frac{1}{4.5}=0\)

\(x-\left(\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}\right)=0\)

\(x-\left(1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}\right)=0\)

\(x-\left(1-\frac{1}{5}\right)=0\)

\(x-\frac{4}{5}=0\)

\(x=\frac{4}{5}\)

\(x-\frac{1}{1.2}-\frac{1}{2.3}-\frac{1}{3.4}=\frac{1}{4.5}\)

\(x-\left(1-\frac{1}{2}\right)-\left(\frac{1}{2}-\frac{1}{3}\right)-\left(\frac{1}{3}-\frac{1}{4}\right)=\frac{1}{4}-\frac{1}{5}\)

\(x-1+\frac{1}{2}-\frac{1}{2}+\frac{1}{3}-\frac{1}{3}+\frac{1}{4}-\frac{1}{4}=\frac{-1}{5}\)

\(x-1=-\frac{1}{5}\)

\(x=\frac{4}{5}\)

\(x-\frac{1}{1.2}-\frac{1}{2.3}-\frac{1}{3.4}=\frac{1}{4.5}\)

\(\Rightarrow x-\text{​​}\text{​​}\frac{1}{1.2}-\frac{1}{2.3}-\frac{1}{3.4}-\frac{1}{4.5}=0\)

\(\Rightarrow x-\left(\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}\right)=0\)

\(\Rightarrow x-\left(1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}\right)=0\)

\(\Rightarrow x-\left(1-\frac{1}{5}\right)=0\)

\(\Rightarrow x-\frac{4}{5}=0\)

\(\Rightarrow x=\frac{4}{5}\)

Vậy  \(x=\frac{4}{5}\)

_Chúc bạn học tốt_

1.2+2.3+3.4+4.5+...+99.100

=1.2.3+2.3.3+3.4.3+4.5.3+...+99.100.3

=1.2.3+2.3.(4-1)+3.4.(5-2)+4.5.(6-3)+...+99.100.(101-98)

=1.2.3+2.3.4-1.2.3+3.4.5-2.3.4+4.5.6-3.4.5+...+99.100.101-98.99.100

=1.2.3-1.2.3+2.3.4-2.3.4+3.4.5-3.4.5+4.5.6-4.5.6+...+99.100.101

=99.100.101=999900

=999900:3=333300

D=1.2+3.4+4.5+...+19.20

3D=1.2.(3-0)+2.3.(4-1)+4.5(6-3)+...+19.20.(21-18)

3D=1.2.3-0.1.2+2.3.4-1.2.3+4.5.6-3.4.5+...+19.20.21-18.19.20

3D=(1.2.3+2.3.4+3.4.5+...+19.20.21)-(0.1.2+1.2.3+2.3.4+...+18.19.20)

3D=19.20.21

3D=7890

D=2660