\(\left(\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+....+\frac{1}{8.9}+\frac{1}{9.10}\right)\)

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

\(=1-\frac{1}{10}\)

\(=\frac{10}{10}-\frac{1}{10}=\frac{9}{10}\)

\(\Leftrightarrow\frac{9}{10}.100-\left[\frac{5}{2}:\left(x+\frac{206}{100}\right):\frac{1}{2}\right]=89\)

\(\Leftrightarrow90-\left[\frac{5}{2}:\left(x+\frac{206}{100}\right):\frac{1}{2}\right]=89\)

\(\Leftrightarrow\frac{5}{2}:\left(x+\frac{206}{100}\right):\frac{1}{2}=90-89=1\)

\(\Leftrightarrow\frac{5}{2}:\left(x+\frac{206}{100}\right)=1.\frac{1}{2}=\frac{1}{2}\)

\(\Leftrightarrow x+\frac{206}{100}=\frac{5}{2}:\frac{1}{2}\)

\(\Leftrightarrow x+\frac{103}{50}=\frac{5}{2}.2\)

\(\Leftrightarrow x+\frac{103}{50}=5\)

\(\Leftrightarrow x=5-\frac{103}{50}\)

\(\Leftrightarrow x=\frac{250}{50}-\frac{103}{50}\)

\(\Leftrightarrow x=\frac{147}{50}\)

a) (-18)+(-31)+98+|-18|+(-69)

((-18)+98)+((-31)+18+(-69))

80+ (-82)

-2

b)15×(-176)+15×76+100×15

=15×((-176)+76+10)

=15×0

=0

c)79×89-79×(-11)-100×79

=79×(89-(-11)-100)

=79×0

=0

Chúc bạn học tốt

a) (-18)+(-31)+98+|-18|+(-69)

=(-18)+(-31)+98+18+(-69)

=[(-18)+18]+[(-31)+(-69)]+98

=0+(-100)+98

=-2

b) 15.(-176)+15.76+100.15

=15[(-176)+76+100]

=15.0=0

c) 79.89-79.(-11)-100.79

=19.[89-(-11)-100]

=19.0=0

147/50

bài 5 : 

21+68+279+132

=(21+279)+(68+132)

=300+200

=500

35.17+84.35-35

=35.17+84.35-35.1

=35.(17+84-1)

=35.100

=3500

136.75+75.64

=(136+64).75

=200.75

=15000

kb ok

mk cần cách giải gấp mấy bạn giúp mk nha

a) Ta có: (-2)*x-49=71

\(\Leftrightarrow-2x=71+49=120\)

hay x=-60

Vậy: x=-60

b) Ta có: (-2)*x-46=68

\(\Leftrightarrow-2x=114\)

hay x=-57

Vậy: x=-57

c) Ta có: \(-2x+167=89\)

\(\Leftrightarrow-2x=-78\)

hay x=39

Vậy: x=39