(1/11-1/13+1/13-1/15+.....+1/19-1/21)*462-y=19

(1/11-1/21)*462-y=19

10/231*462-y=19

20-y=19

y=1

(11×132​+13×152​+15×172​+17×192​+19×212​)×462−y=19

⇒(111−113+113−115+115−117+117−119+119−121)×462−�=19⇒(111​−131​+131​−151​+151​−171​+171​−191​+191​−211​)×462−y=19

⇒(111−121)×462−�=19⇒10231×462−�=19⇒(111​−211​)×462−y=19⇒23110​×462−y=19

$\Rightarrow 20-y=19\Rightarrow y=20-19=1$

Tìm \(x\) biết: 

(\(\dfrac{2}{11\times13}\) + \(\dfrac{2}{13\times15}\)+...+ \(\dfrac{2}{19\times21}\)) x 462 - (204: (\(x-105\)):12) =19

(\(\dfrac{1}{11}-\dfrac{1}{13}\)+ \(\dfrac{1}{13}\) - \(\dfrac{1}{15}\)+...+ \(\dfrac{1}{19}\) - \(\dfrac{1}{21}\)) x 462 - (204 : (\(x\) -105):12) =19

(\(\dfrac{1}{11}\) - \(\dfrac{1}{21}\)) x 462 -  \(\dfrac{204:(x-105)}{12}\) = 19

\(\dfrac{10}{213}\) x 462 - \(\dfrac{204:\left(x-105\right)}{12}\) = 19

20 - \(\dfrac{204:\left(x-105\right)}{12}\) = 19

204:(\(x\) - 105):12 = 20 - 19

204:(\(x-105\)):12 = 1

204: (\(x\) - 105) = 12

          \(x-105\) = 204: 12

           \(x\) - 105 = 17

           \(x\)          = 17 + 105

           \(x\)         = 122

Câu1:                     750

Câu2:                     \(\approx\)5240,322

hơi tốn giấy đó bạn 

bạn đừng có làm hao giấy thế chứ

2/(11x13) = 1/11 - 1/13 ; 2/(13x15) = 1/13 - 1/15 ; 2/(15x17) = 1/15 - 1/17 ; 2/(17x19) = 1/17 - 1/19 ; 2/(19x21) = 1/19 - 1/21

2/(11x13) + 2/(13x15) + 2/(15x17) + 2/(17x19) 2/(19x21) =

1/11 - 1/13 + 1/13 - 1/15 + 1/15 - 1/17 + 1/17 - 1/19 + 1/19 - 1/21 = 1/11 - 1/21 = 101231

Hay:   10/231 x 462 - X = 19

20 - X = 19

X = 20 - 19

X = 1,đó là phần b

3/5x+2/3x=-76 <=> 19/15x=-76 <=> x=-60,đó là phần a

\(\left(\frac{2}{11.13}+\frac{2}{13.15}+...+\frac{2}{19.21}\right)-x+\frac{221}{231}=\frac{4}{3}\)

Ta có: 

\(\left(\frac{1}{11}-\frac{1}{13}+\frac{1}{13}-\frac{1}{15}+....+\frac{1}{19}-\frac{1}{21}\right)-x+\frac{221}{231}=\frac{4}{3}\)

\(\left(\frac{1}{11}-\frac{1}{21}\right)-x+\frac{221}{231}=\frac{4}{3}\)

 \(\frac{10}{231}-x+\frac{221}{231}=\frac{4}{3}\)

\(\frac{10}{231}-x=\frac{4}{3}-\frac{221}{231}=\frac{29}{77}\)

\(x=\frac{10}{231}-\frac{29}{77}\)

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

kết quả là :

    \(\frac{1}{3}\)

        ds...

=1/11 - 1/13 + 1/13 - 1/15 +...+1/19 - 1/21

=1/11-1/21=10/231