\(\left(\frac{1}{3}+\frac{1}{15}+\frac{1}{35}+\frac{1}{63}+\frac{1}{99}\right).y=\frac{2}{3}\)

\(\frac{1}{2}.\left(1-\frac{1}{3}\right)+\frac{1}{2}.\left(\frac{1}{3}-\frac{1}{5}\right)+\frac{1}{2}.\left(\frac{1}{5}-\frac{1}{7}\right)+\frac{1}{2}.\left(\frac{1}{7}-\frac{1}{9}\right)+\frac{1}{2}.\left(\frac{1}{9}-\frac{1}{11}\right).y=\frac{2}{3}\)

\(\frac{1}{2}.\left(1-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{9}+\frac{1}{9}-\frac{1}{11}\right).y=\frac{2}{3}\)

\(\frac{1}{2}.\left(1-\frac{1}{11}\right).y=\frac{2}{3}\)

\(\left(1-\frac{1}{11}\right).y=\frac{4}{3}\)

\(\frac{10}{11}.y=\frac{4}{3}\)

\(\Rightarrow y=\frac{22}{15}\)

ê bạn ơi tìm y sao lại có x

\(\frac{2}{3}\cdot y-\frac{12}{3}:\left(\frac{2}{3}+\frac{2}{15}+\frac{2}{35}+\frac{2}{63}+\frac{2}{99}+\frac{2}{143}\right)=\frac{1}{3}\)\(\frac{1}{3}\)

\(\frac{2}{3}\cdot y-4:\left(\frac{2}{1\cdot3}+\frac{2}{3\cdot5}+\frac{2}{5\cdot7}+\frac{2}{7\cdot9}+\frac{2}{9\cdot11}+\frac{2}{11\cdot13}\right)=\frac{1}{3}\)

\(\frac{2}{3}\cdot y-4:\left(\frac{3-1}{1\cdot3}+\frac{5-3}{3\cdot5}+\frac{7-5}{5\cdot7}+\frac{9-7}{7\cdot9}+\frac{11-9}{9\cdot11}+\frac{13-11}{11\cdot13}\right)=\frac{1}{3}\)

\(\frac{2}{3}\cdot y-4:\left(1+\frac{1}{3}-\frac{1}{3}+\frac{1}{5}-\frac{1}{5}+\frac{1}{7}-\frac{1}{7}+\frac{1}{9}-\frac{1}{9}+\frac{1}{11}-\frac{1}{11}+\frac{1}{13}\right)\)\(=\frac{1}{3}\)

\(\frac{2}{3}\cdot y-4:\left(\frac{1}{1}+\frac{1}{3}\right)=\frac{1}{3}\)

\(\frac{2}{3}\cdot y-4:\frac{4}{3}\)\(=\frac{1}{3}\)

\(\frac{2}{3}\cdot y-4\cdot\frac{3}{4}=\frac{1}{3}\)

\(\frac{2}{3}\cdot y-3=\frac{1}{3}\)

\(\frac{2}{3}\cdot y=\frac{1}{3}+3\)

\(\frac{2}{3}\cdot y=\frac{10}{3}\)

\(y=\frac{10}{3}:\frac{2}{3}\)

y=5

Ta có : \(\frac{x}{4}=\frac{y}{7}\) 

\(\Rightarrow\frac{x^2}{16}=\frac{y^2}{49}=\frac{3x^2}{48}=\frac{4y^2}{196}=\frac{3x^2-4y^2}{48-196}=\frac{100}{-148}=-\frac{25}{37}\)

Thay vào là ra nhé !:D

Cái chỗ Nguyễn Quang Trung đúng ròi

\(\Rightarrow\hept{\begin{cases}\frac{x}{4}=-\frac{25}{37}\\\frac{y}{7}=-\frac{25}{37}\end{cases}}\Rightarrow\hept{\begin{cases}x=-\frac{100}{37}\\y=-\frac{175}{37}\end{cases}}\)

A) x/y-3/8=1                  x/y-3/8=1/2                          B)4/9:x/y=1                                                    4/9:x/y=2/3 

x/y=1+3/8                     x/y=1/2+3/8                                   x/y=4/9:1                                                    x/y=4/9:2/3

x/y=8/8+3/8                  x/y=4/8+3/8                                   x/y=4/9                                                       x/y=2/3

x/y=11/8                       x/y=7/8

a   x/y=1 và 1/2+3/8

x/y=15/8

b   x/y=4/9:1 và 2/3

x/y=4/15

y x 6 + y x 3 + y = 80

y x 6 + y x 3 + y x 1 = 80

y x (6+3+1)=80

y x 10 =80

y        =80:10

y       =8

a) \(y\times6+y\times3+y=80\)

\(\Leftrightarrow y\times\left(6+3+1\right)=80\)

\(\Leftrightarrow y\times10=80\)

\(\Leftrightarrow y=80\div10\)

\(\Leftrightarrow y=8\)

b) \(y+y\div0,5+y\div0,25+y\div0,125=15\)

\(\Leftrightarrow y+y\times2+y\times4+y\times8=15\)

\(\Leftrightarrow y\times\left(1+2+4+8\right)=15\)

\(\Leftrightarrow y\times15=15\)

\(\Leftrightarrow y=1\)

hi

\(\left(y+1\right)+\left(y+2\right)+\left(y+3\right)+...+\left(y+99\right)=2550\)

Có \(\left(99-1\right):1+1=99\)cặp

\(\left(y+y+y+...+y\right)+\left(1+2+3+...+99\right)=2550\)

\(99y+\left[\left(99+1\right)\times99:2\right]=2550\)

\(99y+4590=2550\)

\(99y=2550-4590\)

\(99y=-2040\)

\(y=\frac{-680}{33}\)

Đề hình như sai bạn ơi

#)Giải :

\(\left(y+1\right)+\left(y+2\right)+\left(y+3\right)+...+\left(y+99\right)=2550\)

\(\Rightarrow\left(y+y+y+...+y\right)+\left(1+2+3+...+99\right)=2550\)

\(\Rightarrow99y+\frac{\left(99+1\right).99}{2}=2550\)

\(\Rightarrow99y+4950=2550\)

\(\Leftrightarrow99y=-2400\)

\(\Leftrightarrow y=-24,2424242424.................\approx-24,24\)

P/s : Có j đó sai sai :v đề lỗi ak ?