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\(C=\frac{2}{1\cdot5}+\frac{2}{5\cdot9}+\frac{2}{9\cdot13}+\frac{2}{13\cdot17}+\frac{2}{17\cdot21}\)
\(C=\frac{2}{4}.\left(\frac{4}{1.5}+\frac{4}{5.9}+\frac{4}{9.13}+\frac{4}{13.17}+\frac{4}{17.21}\right)\)
\(=\frac{1}{2}.\left(1-\frac{1}{5}+\frac{1}{5}-\frac{1}{9}+....+\frac{1}{17}-\frac{1}{21}\right)\)
\(=\frac{1}{2}.\left(1-\frac{1}{21}\right)\)
\(=\frac{1}{2}.\frac{20}{21}\)
\(=\frac{10}{21}\)
c; 17\(\dfrac{2}{31}\) - (\(\dfrac{15}{17}\) + 6\(\dfrac{2}{31}\))
= 17 + \(\dfrac{2}{31}\) - \(\dfrac{15}{17}\) - 6 - \(\dfrac{2}{31}\)
= (17 - 6) - \(\dfrac{15}{17}\) + (\(\dfrac{2}{31}\) - \(\dfrac{2}{31}\))
= 11 - \(\dfrac{15}{17}\)+ 0
= \(\dfrac{172}{17}\)
b; 130\(\dfrac{25}{28}\) + 120\(\dfrac{17}{35}\)
= 130 + \(\dfrac{25}{28}\) + 120 + \(\dfrac{17}{35}\)
= (130 + 120) + (\(\dfrac{25}{28}\) + \(\dfrac{17}{35}\))
= 250 + (\(\dfrac{125}{140}\) + \(\dfrac{68}{140}\))
= 250 + \(\dfrac{193}{140}\)
= 250\(\dfrac{193}{140}\)
a) 10*(x-1)=15-5
10*(x-1)=10
(x-1)=10:10
(x-1)=1
x=1+1
x=2
b) x+1+x+2+x+3=18
3x+6=18
3x=18-6
3x=12
x=12:3
x=4
c) 17+2*(x+2)=25
2*(x+2)=25-17
2*(x+2)=8
x+2=8:2
x+2=4
x=4-2
x=2
d) x:17=82-17+26
x:17=91
x=91*17
x=1547
e) 147*x=441
x=441:147
x=3
g)(x:81)=330:10:11
(x:81)=3
x=3*81
x=243
i)x+1+x+2+x+3+x+4+x+5=40
5x+15=40
5x=40-15
5x=25
x=5
Ủng hộ nha
bạn làm sao ấy đề mink viet khac ma ban vite mot neo
Đề?
\(C=\frac{2}{1.7}+\frac{2}{7.13}+...+\frac{2}{601.607}\)
\(C=\frac{2}{6}.\left(1-\frac{1}{7}+\frac{1}{7}-\frac{1}{13}+...+\frac{1}{601}-\frac{1}{607}\right)\)
\(C=\frac{1}{3}.\left(1-\frac{1}{607}\right)\)
\(C=\frac{1}{3}.\frac{606}{607}\)
\(C=\frac{202}{607}\)
Dấu " ." là dấu nhân