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Câu b:
\(\frac{21}{8}:\frac{5}{6}+\frac{1}{2}:\frac{5}{6}\)
= \(\frac{63}{20}+\frac{3}{5}\)
= \(\frac{15}{4}\)
\(\left(\frac{21}{8}+\frac{1}{2}\right):\frac{5}{6}\)
\(\frac{25}{8}:\frac{5}{6}\)
\(\frac{25}{8}.\frac{6}{5}\)
\(\frac{30}{8}\)
1) 9 x 10 + 10 x 11 + 11 x 12 + ....+ 2015 x 2016
=9x10x11-8x9x10+10x11x12-9x10x11+...+2015x2016x2017-2014x2015x2013
=2015x2016x2017-8x9x10
=8193537360
2/3 . x + 3/4 = 3
2/3 . x = 3 - 3/4
2/3 . x = 1/4
x = 1/4 : 2/3
x = 3/8
720 : {41-[2x . (-5)]} =120
41-[2x . (-5)] = 720 : 120
41-[2x . (-5)] = 6
2x . (-5) = 41 - 6
2x . (-5) = 35
2x = 35 :(-5)
2.x=-7
x=-7:2
x=-7/2=-3,5
chúc bạn học tốt nha
ủng hộ mk với nha
Bài làm:
(2019-2018+2017-.....-2) x (100 -25x2x2)
=(2019-2018+2017-.....-2) x (100 -25x4)
=(2019-2018+2017-.....-2) x 0
=0
*like phát
=(2019 – 2018 + 2017 – 2016 + 2015 + ....... – 4 + 3 – 2) x(100-25x4)
=(2019 – 2018 + 2017 – 2016 + 2015 + ....... – 4 + 3 – 2) x(100-100)
=(2019 – 2018 + 2017 – 2016 + 2015 + ....... – 4 + 3 – 2) x0
=0
B=1 x 2 + 2 x 3 + 3 x 4 + 4 x 5 +5 x 6 + ........... + 2015 x 2016
3xB=1 x 2x3 + 2 x 3x3 + 3 x 4x3 + 4 x 5x3 +5 x 6x3 + ........... + 2015 x 2016x3
=1x2x3+2x3x(4-1)+3x4x(5-2)+4x5x(6-3)+...+2014x2015(2016-2013)+2015x2016(2017-2014)
=1x2x3+2x3x4-1x2x3+3x4x5-2x3x4+4x5x6-3x4x5+..+2014x2015x2016-2013x2014x2015+2015x2016x2017-2014x2015x2016
các số hạng tự triệt tiêu nhau
còn lại (2015x2016x2017)
B= (2015x2016x2017)/3=2015x672x2017
2015=5.43
672=.3.4.7.8
=> B chia hết cho 2,4,6,7,8
không chia hết cho 9
Chỉ làm bài khó thôi nhé:::::::::::::::
\(\frac{1}{3}+\frac{1}{6}+\frac{1}{10}+...+\frac{2}{x.\left(x+1\right)}=\frac{2016}{2018}\)
\(\Rightarrow\frac{2}{6}+\frac{2}{12}+\frac{2}{20}+....+\frac{2}{x.\left(x+1\right)}=\frac{2016}{2018}\)
\(\Rightarrow2.\left(\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+...+\frac{1}{x.\left(x+1\right)}\right)=\frac{2016}{2018}\)
\(\Rightarrow\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+...+\frac{1}{x.\left(x+1\right)}=\frac{1013}{2018}\)
\(\Rightarrow\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{x}-\frac{1}{x+1}=\frac{1013}{2018}\)
\(\Rightarrow\frac{1}{2}-\frac{1}{x+1}=\frac{1013}{2018}\)
\(\Rightarrow\frac{1}{x+1}=\frac{1}{2018}\Rightarrow x+1=2018\Rightarrow x=2017\)