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Bài 1: <Cho là câu a đi>:
a. \(\frac{1}{2}+\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{x\left(x+1\right)}=\frac{49}{50}\)
\(\rightarrow\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{x\left(x+1\right)}=\frac{49}{50}\)
\(\rightarrow1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+...+\frac{1}{x}-\frac{1}{x+1}=\frac{49}{50}\)
\(\rightarrow1-\frac{1}{x+1}=\frac{49}{50}\)
\(\rightarrow\frac{1}{x+1}=1-\frac{49}{50}=\frac{1}{50}\)
\(\rightarrow x+1=50\rightarrow x=49\)
Vậy x = 49.
\(a)\frac{x}{8}=\frac{-30}{y}=\frac{-48}{32}\)
Rút gọn : \(\frac{-48}{32}=\frac{(-48):16}{32:16}=\frac{-3}{2}\)
* Ta có : \(\frac{x}{8}=\frac{-3}{2}\)
\(\Rightarrow x\cdot2=-3\cdot8\)
\(\Rightarrow x=\frac{-3\cdot8}{2}=-12\)
* Ta có : \(\frac{-30}{y}=\frac{-3}{2}\)
\(\Rightarrow-30\cdot2=-3\cdot y\)
\(\Rightarrow y=\frac{-30\cdot2}{-3}=20\)
Mấy bài kia làm tương tự
a) \(22\frac{1}{2}\cdot\frac{7}{9}+50\%-1,25\)
\(=\frac{45}{2}\cdot\frac{7}{9}+\frac{50}{100}-\frac{125}{100}\)
\(=\frac{5}{2}\cdot\frac{7}{1}+\frac{1}{2}-\frac{5}{4}\)
\(=\frac{35}{2}+\frac{1}{2}-\frac{5}{4}=18-\frac{5}{4}=\frac{67}{4}\)
b) \(1,4\cdot\frac{15}{49}-\left(\frac{4}{5}+\frac{2}{3}\right):2\frac{1}{5}\)
\(=\frac{7}{5}\cdot\frac{15}{49}-\frac{22}{15}:\frac{11}{15}\)
\(=\frac{1}{1}\cdot\frac{3}{7}-\frac{22}{15}\cdot\frac{15}{11}\)
\(=\frac{3}{7}-2=\frac{3-14}{7}=\frac{-11}{7}\)
c) \(\left(-\frac{1}{2}\right)^2-\frac{7}{16}:\frac{7}{4}+75\%\)
\(=\frac{1}{4}-\frac{7}{16}\cdot\frac{4}{7}+\frac{75}{100}\)
\(=\frac{1}{4}-\frac{1}{4}+\frac{3}{4}=\frac{3}{4}\)
Bài 2 Bạn tự làm nhé
1.a,\(22\frac{1}{2}.\frac{7}{9}+50\%-1,25\)
\(=\frac{45}{2}.\frac{7}{9}+\frac{1}{2}-\frac{5}{4}\)
\(=\frac{35}{2}+\frac{1}{2}-\frac{5}{4}\)
\(=\frac{67}{4}\)
b,Các phép tính khác làm tương tự
Đổi các số ra hết thành phân số,có ngoặc thì lm ngoặc trc,Xoq đến nhân chia trước dồi mới cộng trừ
c,tương tự
2.
a,\(1\frac{3}{5}+\frac{7}{12}\div x=\frac{-9}{4}\)
\(\frac{8}{5}+\frac{7}{12}\div x=\frac{-9}{4}\)
\(\frac{7}{12}\div x=\frac{-77}{20}\)
Đến đây dễ bạn tự làm
b,\(\left(2\frac{4}{5}.x+50\right)\div\frac{2}{3}=-51\)
\(\left(\frac{14}{5}x+50\right)\div\frac{2}{3}=-51\)
\(\frac{14}{5}x+50=-34\)
\(\frac{14}{5}x=-84\)
Tự làm tiếp
c,\(\left|\frac{3}{4}.x-\frac{1}{2}\right|=\frac{1}{4}\)\(\Rightarrow\left|\frac{3}{4}x-\frac{1}{2}\right|=\varnothing\)
\(a,\frac{3}{4}.\left(x+2\right)+\frac{1}{2}.\left(x-\frac{1}{2}\right)=\frac{15}{4}\)
\(\frac{3}{4}.x+\frac{3}{4}.2+\frac{1}{2}.x+\frac{1}{2}.\left(-\frac{1}{2}\right)=\frac{15}{4}\)
\(\left(\frac{3}{4}.x+\frac{1}{2}.x\right)+\frac{3}{2}-\frac{1}{4}=\frac{15}{4}\)
\(\left(\frac{3}{4}+\frac{1}{3}\right).x=\frac{15}{4}+\frac{1}{4}-\frac{3}{2}\)
\(\frac{5}{4}.x=\frac{5}{2}\)
\(x=\frac{5}{2}:\frac{5}{4}\)
\(x=2\)
\(b,3.x-\frac{3}{5}=0\)
\(3.x=0+\frac{3}{5}\)
\(3.x=\frac{3}{5}\)
\(x=\frac{3}{5}:3\)
\(x=\frac{1}{5}\)
\(c,\frac{-2}{3}.x-\frac{1}{3}.\left(2.x-3\right)=\frac{3}{2}\)
\(\frac{-2}{3}.x-\frac{2}{3}.x+1=\frac{3}{2}\)
\(\left(\frac{-2}{3}-\frac{2}{3}\right).x=\frac{3}{2}-1\)
\(-\frac{4}{3}.x=\frac{1}{2}\)
\(x=\frac{1}{2}:\left(\frac{-4}{3}\right)\)
\(x=\frac{-3}{8}\)
Học tốt
a)(1,5.x+3/7):1,5-1,5=1 b)(2+x+4/7+3/7):1,4-5/7=-2
1,5.x:1,5+3/7:3/2=1+1,5 (2+x+1):1,4-5/7=-2
x+ 3/7x 2/3=2,5 (3+x):1,4=-2+5
x+2/7=2,5 (3+x):1,4=3
x+2/7=5/2 3+x=3x1,4
x=5/2-2/7 3+x=4,2
x=31/14 x=4,2-3=1,2
a) \(\left(\frac{5}{6}-\frac{2}{3}\right)+\frac{1}{4}:x=-4\)
\(\Rightarrow\frac{1}{6}+\frac{1}{4}:x=-4\)
\(\Rightarrow\frac{1}{4}:x=-4-\frac{1}{6}\)
\(\Rightarrow\frac{1}{4}:x=-\frac{25}{6}\)
\(\Rightarrow\)\(x=\frac{1}{4}:-\frac{25}{6}\)
\(\Rightarrow x=-\frac{3}{50}\)
b) \(\left|2x-\frac{1}{3}\right|+1=\frac{5}{6}\)
\(\Rightarrow\left|2x-\frac{1}{3}\right|=\frac{5}{6}-1\)
\(\Rightarrow\left|2x-\frac{1}{3}\right|=-\frac{1}{6}\)
\(\Rightarrow\orbr{\begin{cases}2x-\frac{1}{3}=-\frac{1}{6}\\2x-\frac{1}{3}=\frac{1}{6}\end{cases}}\)
\(\Rightarrow\orbr{\begin{cases}2x=-\frac{1}{6}+\frac{1}{3}\\2x=\frac{1}{6}+\frac{1}{3}\end{cases}}\)
\(\Rightarrow\orbr{\begin{cases}2x=\frac{1}{6}\\2x=\frac{1}{2}\end{cases}}\)
\(\Rightarrow\orbr{\begin{cases}x=\frac{1}{6}:2\\x=\frac{1}{2}:2\end{cases}}\)
\(\Rightarrow\orbr{\begin{cases}x=\frac{1}{12}\\x=\frac{1}{4}\end{cases}}\)
\(\left(3x-1\right)⋮\left(x+1\right)\)
\(\Rightarrow\left(3x+3-4\right)⋮\left(x+1\right)\)
\(\Rightarrow\left(-4\right)⋮\left(x+1\right)\)
\(\Rightarrow x+1\inƯ\left(-4\right)=\left\{-4;-1;1;4\right\}\)
\(\Rightarrow x\in\left\{-5;-2;0;3\right\}\)
\(\frac{x}{3}\cdot\frac{4}{2}-\frac{x}{3}\cdot\frac{1}{3}=\frac{1}{2}\)
\(\frac{x}{3}\cdot\left(\frac{4}{2}-\frac{1}{3}\right)=\frac{1}{2}\)
\(\frac{x}{3}\cdot\frac{5}{3}=\frac{1}{2}\)
\(\frac{x}{3}=\frac{1}{2}\div\frac{5}{3}\)
\(\frac{x}{3}=\frac{3}{10}\)
\(\Rightarrow x\cdot10=3\cdot3\)
\(x=\frac{9}{10}\)
= âm 4,8