1 )  tính nhanh

 17 x 115 + 15 x 17 -120

=17.(115+15)-120

=17.130-120

=2210-120

=2090

2) tìm X

a) 140 :5 (X - 8 ) = 7

5(x-8)=20

x-8=4

x=12

Bài 1 :

17x115+15x17-120

=17x(115+15)-120

=17x130-120

=2210-120

=2090

Bài 2 :

140:5(x-8)=7

28(x-8)=7

x-8=\(\frac{7}{28}\)

x=\(\frac{33}{4}\)

#H

M = 5 + 5² + 5³ + ... + 5²⁰¹²

M = ( 5+1 ).5² + ( 5+1 ). 5³ +...+( 5+1 ). 5 ⁸⁰

M = 6. 5² + 6. 5³ + ...+ 6. 5 ⁸⁰

M = 6.( 5²+.5³+...+5 ⁸⁰ ) ⁝ 6 ( do 5²+.5³+...+5 ⁸⁰ ∈ N )

Vậy M ⁝ 6

\(\frac{-5}{9}\) .\(\frac{13}{28}\) - \(\frac{13}{28}\) . \(\frac{4}{9}\) 

 13/ 28 .( -5/9 - 4/9)

 13/28 .-1

-13/28

A) \(\frac{1}{3}+\frac{8}{15}=\frac{5}{15}+\frac{8}{15}=\frac{13}{15}\)

B) \(\frac{9}{2}-\frac{3}{8}=\frac{36}{8}-\frac{3}{8}=\frac{33}{8}\)

C) \(\frac{15}{12}\times\frac{36}{40}=\frac{540}{480}=\frac{9}{8}\)

D) \(\frac{4}{5}\div\frac{6}{15}=\frac{4}{5}\times\frac{15}{6}=\frac{12}{6}=\frac{2}{1}=2\)

\(\text{Vì }\hept{\begin{cases}560⋮7\\18+3⋮7\end{cases}\Rightarrow560+18+3⋮7}\)

\(\text{~~Học tốt~~}\)

Đáp án:

Ta có 560 + 18 + 3 = 560 + 21

Mà 560 ⋮ 7 và 21⋮ 7 ⇒ (560 + 18 + 3) ⋮ 7

Hok tốt

\(\frac{22}{5}\)\(\div\)\(\left(\frac{-33}{10}\right)\)\(+\)\(x\)\(=\)\(\frac{-1}{6}\)

\(\left(\frac{-33}{10}\right)\)\(+\)\(x\)\(=\)\(\frac{22}{5}\)\(\div\)\(\frac{-1}{6}\)

\(\left(\frac{-33}{10}\right)\)\(+\)\(x\)\(=\)\(\frac{-132}{5}\)

\(x\)\(=\)\(\frac{-132}{5}\)\(-\)\(\left(\frac{-33}{10}\right)\)

\(x\)\(=\)\(\frac{-132}{5}\)\(+\)\(\frac{33}{10}\)

\(x\)\(=\)\(\frac{-231}{10}\)

Hok tốt

\(\frac{22}{5}\div\left(\frac{-33}{10}\right)+x=\frac{-1}{6}\)

\(\Rightarrow\left(\frac{-33}{10}\right)+x=\frac{22}{5}\div\frac{-1}{6}=\frac{-132}{5}\)

\(\Rightarrow x=\frac{-132}{5}-\left(\frac{-33}{10}\right)=\frac{-231}{10}\)

 \(\text{Vậy }x=\frac{-231}{10}\)

lấy 120*3/4=90

Đáp án: 90 nha bạn

\(\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+..+\frac{1}{2021.2022}\)

\(=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-.....+\frac{1}{2021}-\frac{1}{2022}\)

\(=1-\frac{1}{2022}\)

\(=\frac{2021}{2022}\)

\(\frac{1}{1.2}\)\(+\)\(\frac{1}{2.3}\)\(+\)\(\frac{1}{3.4}\)\(+\)\(...+\)\(\frac{1}{2021.2022}\)

\(=\)\(1-\frac{1}{2}\)\(+\)\(\frac{1}{2}\)\(-\)\(\frac{1}{3}\)\(+\)\(\frac{1}{3}\)\(-\)\(\frac{1}{4}\)\(+\)\(...+\)\(\frac{1}{2021}\)\(-\)\(\frac{1}{2022}\)

\(=\)\(1\)\(-\)\(\frac{1}{2022}\)

\(=\)\(\frac{2021}{2022}\)

Hok tốt