\(a,=\frac{7-1}{1.3.7}+\frac{9-3}{3.7.9}+\frac{13-7}{7.9.13}+\frac{15-9}{9.13.15}\)\(+\frac{19-13}{13.15.19}\)

\(=\frac{1}{1.3}-\frac{1}{3.7}+\frac{1}{3.7}-\frac{1}{7.9}+\frac{1}{7.9}-\frac{1}{9.13}+\frac{1}{9.13}-\frac{1}{13.15}+\frac{1}{13.15}-\frac{1}{15.19}\)

\(=\frac{1}{1.3}-\frac{1}{15.19}=\frac{95}{285}-\frac{1}{285}=\frac{94}{285}\)

\(b,=\frac{1}{6}.\left(\frac{6}{1.3.7}+\frac{6}{3.7.9}+\frac{6}{7.9.13}+\frac{6}{9.13.15}+\frac{6}{13.15.19}\right)\)

làm giống như trên

\(c,=\frac{1}{8}.\left(\frac{1}{1.2.3}+\frac{1}{2.3.4}+\frac{1}{3.4.5}+...+\frac{1}{48.49.50}\right)\)

\(=\frac{1}{16}.\left(\frac{2}{1.2.3}+\frac{2}{2.3.4}+\frac{2}{3.4.5}+...+\frac{2}{48.49.50}\right)\)

\(=\frac{1}{16}.\left(\frac{3-1}{1.2.3}+\frac{4-2}{2.3.4}+\frac{5-3}{3.4.5}+...+\frac{50-48}{48.49.50}\right)\)

\(=\frac{1}{16}.\left(\frac{1}{1.2}-\frac{1}{2.3}+\frac{1}{2.3}-\frac{1}{3.4}+\frac{1}{3.4}-\frac{1}{4.5}+...+\frac{1}{48.49}-\frac{1}{49.50}\right)\)

\(=\frac{1}{16}.\left(\frac{1}{2}-\frac{1}{2450}\right)=\frac{1}{16}.\left(\frac{1225}{2450}-\frac{1}{2450}\right)=\frac{153}{4900}\)

\(d,=\frac{5}{7}.\left(\frac{7}{1.5.8}+\frac{7}{5.8.12}+\frac{7}{8.12.15}+...+\frac{7}{33.36.40}\right)\)

\(=\frac{5}{7}.\left(\frac{8-1}{1.5.8}+\frac{12-5}{5.8.12}+\frac{15-8}{8.12.15}+...+\frac{40-33}{33.36.40}\right)\)

\(=\frac{5}{7}.\left(\frac{1}{1.5}-\frac{1}{5.8}+\frac{1}{5.8}-\frac{1}{8.12}+\frac{1}{8.12}-\frac{1}{12.15}+...+\frac{1}{33.36}-\frac{1}{36.40}\right)\)

\(=\frac{5}{7}.\left(\frac{1}{5}-\frac{1}{1440}\right)=\frac{5}{7}.\left(\frac{288}{1440}-\frac{1}{1440}\right)=\frac{41}{288}\)

P/S: . là nhân nha

sxasxsxxsxsxssxsxsxsxsx232332321322

Cái này dễ thế mà ko bít lm

Ta có :

\(\left(1+\frac{1}{3}+\frac{1}{5}+\frac{1}{7}+\frac{1}{9}\right)-\left(\frac{1}{2}+\frac{1}{4}+\frac{1}{6}+\frac{1}{8}+\frac{1}{10}\right)\)

\(=\left(1+\frac{1}{2}+\frac{1}{3}+\frac{1}{4}+...+\frac{1}{9}+\frac{1}{10}\right)-2\left(\frac{1}{2}+\frac{1}{4}+\frac{1}{6}+\frac{1}{8}+\frac{1}{10}\right)\)

\(=\left(1+\frac{1}{2}+\frac{1}{3}+\frac{1}{4}+...+\frac{1}{9}+\frac{1}{10}\right)-\left(1+\frac{1}{2}+\frac{1}{3}+\frac{1}{4}+\frac{1}{5}\right)\)

\(=\frac{1}{6}+\frac{1}{7}+\frac{1}{8}+\frac{1}{9}+\frac{1}{10}\left(đpcm\right)\)

Không tính kết quả

2 = 1 + 1     6 = 2 + 4     8 = 5 + 3     10 = 8 + 2

3 = 1 + 2     6 = 3 + 3     8 = 4 + 4     10 = 7 + 3

4 = 3 + 1     7 = 1 + 6     9 = 8 + 1     10 = 6 + 4

4 = 2 + 2     7 = 5 + 2     9 = 6+ 3     10 = 5 + 5

5 = 4 + 1     7 = 4 + 3     9 = 7 + 2     10 = 10 + 0

5 = 3 + 2     8 = 7 + 1     9 = 5 + 4     10 = 0 + 10

6 = 5 + 1     8 = 6 + 2     10 = 9 + 1     1 = 1 + 0

Tiếng  việt  khó .

NHẢM NHÍ

215

215

Cái này

`#lv`

`-2/5+(-7/10)-9/6`

`=-2/5-7/10-9/6`

`=-4/10-7/10-9/6`

`=-11/10-3/2`

`=-11/10-15/10`

`=-26/10`

`=-13/5`

__

`1/2+(-2/5)-(-2/3)`

`=1/2-2/5+2/3`

`=5/10-4/10+2/3`

`=1/10+2/3`

`=3/30+20/30`

`=23/30`

__

`5/3-3/4+7/6`

`=20/12-9/12+14/12`

`=11/12+14/12`

`=25/12`

__

`-1/5+5/3-3/2`

`=-3/15+25/15-3/2`

`=22/15-3/2`

`=44/30-45/30`

`=-1/30`

__

`1/4-7/8+(-9/10)`

`=2/8-7/8-9/10`

`=-5/8-9/10`

`=-25/40-36/40`

`=-61/40`

__

`5/4+1/2+(-7/12)`

`=15/12+6/12-7/12`

`=21/12-7/12`

`=14/12`

`=7/6`

__

`-5/8-1/3+(-7/6)`

`=-5/8-1/3-7/6`

`=-15/24-8/24-28/24`

`=-52/24`

`=-13/6`

__

`-3/4-7/10+(-5/6)`

`=-3/4-7/10-5/6`

`=-45/60-42/60-50/60`

`=-137/60`

Bài làm

a, (-2/5) + (-7/10) - 9/6 =(-12/30) + (-21/30) - 45/30=(-33/30) - 45/30 = (-78/30)

b,1/2 + (-2/5) - (-2/3) = 15/30 + (-12/30) - (-20/30) = 3/30 - (-20/30) = 23/30

c,5/3 - 3/4 + 7/6 = 40/24 - 18/24 + 28/24 = 22/24 + 28/24 = 50/24 = 25/12

d, (-1/5) + 5/3 - 3/2 = (-6/30) + 50/30 - 45/30 = 44/30 - 45/30 = (-1/30)

e, 1/4 - 7/8 + (-9/10) = 10/40 - 35/40 - 36/40 = (-25/40) - 36/40 = (-61/40)

f, 5/4 +1/2 + (-7/12) = 15/12 + 6/12 - 7/12 = 21/12 - 7/12 = 14/12 = 7/6

g,(-5/8) -1/3 + (-7/6) = (-15/24) - 8/24 - 28/24 = (-23/24) - 28/24 = (-51/24) 

k,(-3/4) - 7/10 + (-5/6) = (-45/60) - 42/60 - 50/60 = (-87/60) - 50/60 = (-137/60)

chúc bn học tốt nha

sai thì mn góp ý giúp mk