=chịu

tính như bình thường nha bạn. Có những câu đề bài là tính thuận tiện mà tính thường mk đã gặp nhiều rồi

2,140579710144928

1/5.8+1/8.11+....+1/x(x+3)=1/6

=> 3/5.8 + 3/8.11 +.....+ 3/x(x+3)=1/2

=> 1/5-1/x+3=1/2

=> 1/x+3=1/5-1/2=-3/10=1/-10/3

=>x+3=-10/3=>x=-19/3

                   \(\frac{1}{5\cdot8}+\frac{1}{8\cdot11}+...+\frac{1}{x\left(x+3\right)}=\frac{1}{6}\)

                 \(\frac{3}{5\cdot8}+\frac{3}{8\cdot11}+...+\frac{3}{x\cdot\left(x+3\right)}=\frac{1}{2}\)

\(\frac{1}{5}-\frac{1}{8}+\frac{1}{8}-\frac{1}{11}+....+\frac{1}{x}-\frac{1}{x+3}=\frac{1}{2}\)

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

                                                                          \(\frac{1}{x+3}=\frac{-3}{10}\)

\(\Rightarrow\left(-3\right)\left(x+3\right)=10\)

\(\Rightarrow x+3=\frac{-10}{3}\)

\(\Rightarrow x=\frac{-19}{3}\)

Vậy \(x=\frac{-19}{3}\)

a)2/13 . 5 - 9/11 . 2/13 - 7/11 . 2/13

=2/13.(5-9/11-7/11)

=2/13.39/11

=6/11.

b)(-1/2)² + 4,25

=1/4+4,25

=1/4+17/4

=9/2.

c)(-2/3)² : ( --2 2/3) -- ( 5/8 -- 5/6 ) + 5.8--45/5.8

= -4/3 : -8/3 - -5/24 + 40 - 72

= -751/24.

\(\left(x-3\right)^2+\frac{-9}{25}=\frac{2}{5}.\frac{8}{5}\)

\(\left(x-3\right)^2+\frac{-9}{25}=\frac{16}{25}\)

\(\left(x-3\right)^2=\frac{16}{25}+\frac{-9}{25}\)

\(\left(x-3\right)^2=\frac{7}{25}\)

\(\left(x-3\right)^2=\left(\frac{7}{5}\right)^2\)

\(\Rightarrow x-3=\frac{7}{5}\)

\(\Rightarrow x=\frac{7}{5}+3\)

\(\Rightarrow x=\frac{22}{5}\)

\(\left(x-3\right)^2+\left(\frac{-9}{25}\right)=\frac{2}{5}.\frac{8}{5}\)

\(\left(x-3\right)^2+\left(\frac{-9}{25}\right)=\frac{16}{25}\)

\(\left(x-3\right)^2=1=1^2\)

\(\Rightarrow x-3=1\)

\(\Rightarrow x=4\)

P/s: Hoq chắc :<

( x - 3 )²  + \(\frac{-9}{25}=\frac{2}{5}.\frac{8}{5}\)

 ( x - 3 )²  + \(\frac{-9}{25}=\frac{16}{25}\)

( x - 3 ) ²  = \(\frac{16}{25}-\frac{-9}{25}\)

( x - 3 )² = 1² = ( -1 )² 

\(\hept{\begin{cases}x-3=-1\\x-3=1\end{cases}}=>\hept{\begin{cases}x=2\\x=4\end{cases}}\)

vậy x = 2 , 4 

\(D=\dfrac{2}{3}.\dfrac{5}{5.8}+\dfrac{11}{8.19}+\dfrac{13}{19.32}\)

\(D=\dfrac{2.5}{2.5.8}+\dfrac{1}{19}.\dfrac{11}{8}+\dfrac{1}{19}.\dfrac{13}{32}\)

\(D=\dfrac{1}{12}+\dfrac{1}{19}.\left(\dfrac{11}{8}+\dfrac{13}{32}\right)\)

\(D=\dfrac{1}{12}+\dfrac{1}{19}.\left(\dfrac{44}{32}+\dfrac{13}{32}\right)\)

\(D=\dfrac{1}{12}+\dfrac{1}{19}.\dfrac{57}{32}\)

\(D=\dfrac{1}{12}+\dfrac{3}{32}\)

\(D=\dfrac{8}{96}+\dfrac{9}{96}\)

\(D=\dfrac{17}{96}\)