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làm tiếp cái trước(ấn nhầm)
\(x=\frac{5}{42}-\frac{15}{28}\)
\(x=\frac{5.4}{6.4.7}-\frac{15.6}{4.7.6}\)
\(x=\frac{20}{168}-\frac{90}{168}\)
\(x=\frac{-70}{168}\)
\(x=\frac{-5}{12}\)
2.
1.
\(\frac{11}{13}-\left(\frac{5}{42}-x\right)=-\left(\frac{15}{28}-\frac{11}{13}\right)\)
\(\frac{11}{13}-\frac{5}{42}+x=-\frac{15}{28}+\frac{11}{13}\)
\(\frac{11}{13}-\frac{11}{13}-\frac{5}{42}+\frac{15}{28}=-x\)
\(\frac{11}{13}-\left(\frac{5}{42}-x\right)=-\left(\frac{5}{28}-\frac{11}{13}\right)\)
\(\frac{11}{13}-\frac{5}{42}+x=-\frac{5}{28}+\frac{11}{13}\)
\(x=-\frac{5}{28}+\frac{11}{13}-\frac{11}{13}+\frac{5}{42}\)
\(x=\left(\frac{11}{13}-\frac{11}{13}\right)-\left(\frac{5}{28}-\frac{5}{42}\right)\)
\(x=-\left(\frac{15}{84}-\frac{10}{84}\right)\)
\(x=-\frac{5}{84}\)
A) \(\frac{7}{\left(x+3\right)\left(x+10\right)}+\frac{11}{\left(x+10\right)\left(x+21\right)}+\frac{13}{\left(x+21\right)\left(x+34\right)}\)
\(=\frac{\left(x+10\right)-\left(x+3\right)}{\left(x+3\right)\left(x+10\right)}+\frac{\left(x+21\right)-\left(x+10\right)}{\left(x+10\right)\left(x+21\right)}+\frac{\left(x+34\right)-\left(x+21\right)}{\left(x+21\right)\left(x+34\right)}\)
\(=\frac{1}{x+3}-\frac{1}{x+10}+\frac{1}{x+10}-\frac{1}{x+21}+\frac{1}{x+21}-\frac{1}{x+34}\)
\(=\frac{1}{x+3}-\frac{1}{x+34}\)
\(=\frac{\left(x+34\right)-\left(x+3\right)}{\left(x+3\right)\left(x+34\right)}\)\(=\frac{x}{\left(x+3\right)\left(x+34\right)}\)
\(\Rightarrow\left(x+34\right)-\left(x+3\right)=x\)
\(\Rightarrow x=31\)
Vậy, x = 31
Bạn áp dụng: \(\frac{k}{x\cdot\left(x+k\right)}=\frac{1}{x}-\frac{1}{x+k}\) với \(x,k\inℝ;x\ne0;x\ne-k\)
Chứng minh: \(\frac{1}{x}-\frac{1}{x+k}=\frac{x+k}{x\left(x+k\right)}-\frac{x}{x\left(x+k\right)}=\frac{x+k-x}{x\left(x+k\right)}=\frac{k}{x\left(x+k\right)}\)
11/13-(5/42-x)= 243/364
5/42-x=243/364- 11/13
5/42-x= -5/28
x=5/42-(-5/28)
x=25/84
chúc bạn học giỏi
\(\frac{11}{13}-\left(\frac{5}{42}-x\right)=-\left(\frac{5}{28}-\frac{11}{13}\right)\)
\(\frac{11}{13}-\frac{5}{42}+x=-\frac{5}{28}+\frac{11}{13}\)
=> \(x=-\frac{5}{28}+\frac{11}{13}-\frac{11}{13}+\frac{5}{42}\)
=> \(x=-\frac{5}{28}+\frac{5}{42}\)
=> \(x=-\frac{210}{1176}+\frac{140}{1176}=\frac{-70}{1176}=\frac{-5}{84}\)