C

C.75 min

Bài này là của lớp 6 nên bạn học lớp 5 có thể bỏ qua nhé

Tìm y:

\(\frac34\) + \(\frac{3}{28}\) + \(\frac{3}{70}\) + \(\frac{3}{130}\) + ... + \(\frac{3}{y}\) = \(\frac{33}{34}\)

\(\frac{3}{1.4}\) + \(\frac{3}{4.7}\) + \(\frac{3}{7.10}\) + \(\frac{3}{10.13}\) + ... + \(\frac{3}{y}\) = \(\frac{33}{34}\)

Xét dãy số: 1; 4; 7; 10;...

mst1 = 1 = 3 - 2

mst2 = 4 = 3.2 - 2

mst3 = 7 = 3.3 - 2

mst4 = 10 = 3.4 - 2

.................................

stn = 3.n - 2

Xét dãy số: 4; 7; 10;...

mst1 = 4 = 3 + 1

mst2 = 7 = 3.2 + 1

...................................

mstn = (3.n + 1)

VT = \(\frac{3}{1.4}\) + \(\frac{3}{4.7}\) + ... + \(\frac{3}{\left(3n-2\right).\left(3n+1\right)}\)

Theo bài ra ta có: \(\frac{3}{1.4}+\frac{3}{4.7}+\cdots+\frac{3}{\left(3n-2\right)\left(3n+1\right)}\) = \(\frac{33}{34}\)

\(\frac11-\frac14\) + \(\frac14-\frac17\)+ ... + \(\frac{1}{3n-2}\) - \(\frac{1}{3n+1}\) = \(\frac{33}{34}\)

\(\frac11\) - \(\frac{1}{3n+1}\) = \(\frac{33}{34}\)

\(\frac{1}{3n+1}\) = 1 - \(\frac{33}{34}\)

\(\frac{1}{3n+1}\) = \(\frac{1}{34}\)

3n + 1 = 34

3n = 34 - 1

3n = 33

n = 33: 3

n = 11

y = (3n - 2),(3n + 1)

y = (3.11 - 2).(3.11 + 1)

y = (33 - 2).(33 + 1)

y = 31.34

y = 1054

\(\frac34+\frac{3}{28}+\frac{3}{70}+\frac{3}{130}+\ldots+\frac{3}{y}=\frac{33}{34}\)

\(\frac{3}{1.4}+\frac{3}{4.7}+\frac{3}{7.10}+\frac{3}{10.13}+\cdots+\frac{3}{y}=\frac{33}{44}\)

\(\frac{4-1}{1.4}+\frac{7-4}{4.7}+\frac{10-7}{7.10}+\frac{13-10}{10.13}+\cdots+\frac{3}{y}=\frac{33}{34}\)

\(1-\frac14+\frac14-\frac17+\frac17-\frac{1}{10}+\frac{1}{10}-\frac{1}{13}+\cdots+\frac{3}{y}=\frac{33}{34}\)

\(1+\frac{3}{y}=\frac{33}{34}\)

\(\frac{3}{y}=\frac{33}{34}-1\)

\(\frac{3}{y}=\frac{-1}{34}\) \(\Rightarrow y=\frac{3.34}{-1}\)

\(y=-102\)

Vậy \(y=-102\)

`3/4 + 3/28 + 3/70 + 3/130 + ... +3/y = 33/34`

`=> 3/(1*4) + 3/(4*7) + 3/(7*10) + 3/(10*13) + ... + 3/y = 33/34`

`=> 1 - 1/4 + 1/4 - 1/7 + 1/7 - 1/10 + 1/10 - 1/13 + .... + 3/y = 33/34`

`=> 1 + 3/y = 33/34`

`=>        3/y = 33/34 - 1`

`=> 3/y = 33/34 - 34/34`

`=> 3/y = -1/34`

`=> 3 : y = -1/34`

`=>     y = 3: -1/34`

`=>    y = 3 xx -34`

`=> y = -102`

Vậy `y = -102`

`(26x + 1392) xx 15 =2595`

`=> 26x + 1392 = 2592 : 15`

`=> 26x +1392 = 173`

`=> 26x = 173 - 1392`

`=> 26x  =-1219`

`=> x = -1219 : 26`

`=> x = -1219/26`

Vậy `x = -1219/26`

a) (26*x+1392)*15=2595

26*x+1392 =2595:15

26*x+1392 =173

26*x =173-139

26*x =-1219

x =-1219:26

rồi tự tính:)))))))))))))))))))))))

40128 : 24 = 1672

40128:24=1672

  13/8:5/6:3/5-2/3                      

=13/8x6/5x5/3-2/3

=13/8x[6/5x5/3]-2/3

=13/8x2-2/3

=13/4-2/3

=39/12-8/12

=31/12

= 3/7 nha

`3x + 17 =12`

`=> 3x =12 - 17`

`=> 3x = -5`

`=> x = -5/3`

Vậy `x = -5/3`

Ta có: 3x+17=12

=>3x=12-17=-5

=>\(x=-\dfrac{5}{3}\)