Câu 3:
b: Áp dụng tính chất của dãy tỉ số bằng nhau, ta được:
\(\dfrac{a}{2}=\dfrac{b}{3}=\dfrac{c}{4}=\dfrac{2a-b+3c}{2\cdot2-3+3\cdot4}=\dfrac{39}{13}=3\)
Do đó: a=6; b=9; c=12
\(\text{1)Thực hiện phép tính(tính nhanh nếu có thể):}\)
\(\text{a)}\dfrac{5}{21}-1\dfrac{1}{7}+\dfrac{4}{3}\) \(\text{b)}\dfrac{12}{25}+\dfrac{27}{13}+\dfrac{13}{25}-\dfrac{40}{13}-0,25\)
\(=\dfrac{5}{21}+\dfrac{-8}{7}+\dfrac{4}{3}\) \(=\dfrac{12}{25}+\dfrac{27}{13}+\dfrac{13}{25}+\left(\dfrac{-40}{13}\right)+\left(\dfrac{-1}{4}\right)\)
\(=\left(\dfrac{-19}{21}\right)+\dfrac{4}{3}\) \(=\left(\dfrac{12}{25}+\dfrac{13}{25}\right)+\left(\dfrac{27}{13}+\dfrac{-40}{13}\right)+\left(\dfrac{-1}{4}\right)\)
\(=\dfrac{3}{7}\) \(=1+\left(-1\right)+\left(\dfrac{-1}{4}\right)\)
\(=0+\left(\dfrac{-1}{4}\right)=\dfrac{-1}{4}\)
\(\text{c)}1,6.\sqrt{100}+9:\left(\dfrac{-3}{2}\right)^3-\left(\dfrac{2019}{2020}\right)^0\)
\(=\dfrac{8}{5}.10+9:\dfrac{27}{8}-1\)
\(=16+\dfrac{8}{3}-1\)
\(=\dfrac{56}{3}-1=\dfrac{53}{3}\)
\(\text{2)Tìm x,biết:}\)
\(\text{a)}1\dfrac{2}{5}+x=\dfrac{11}{5}\) \(\text{b)}\dfrac{1}{5}x-1\dfrac{2}{3}=\dfrac{4}{3}\)
\(\dfrac{7}{5}+x=\dfrac{11}{5}\) \(\dfrac{1}{5}x\) \(=\dfrac{4}{3}+\dfrac{5}{3}=\dfrac{9}{3}=3\)
\(x=\dfrac{11}{5}+\left(\dfrac{-7}{11}\right)=\dfrac{86}{55}\) \(x\) \(=3.5=15\)
\(\text{c)}\left|x-\dfrac{1}{2}\right|=\dfrac{3}{5}\)
\(\text{Vậy x-}\dfrac{1}{2}=\dfrac{3}{5}\)
\(x\) \(=\dfrac{3}{5}+\left(\dfrac{-1}{2}\right)=\dfrac{1}{10}\)
\(\text{hoặc x-}\dfrac{1}{2}=\left(\dfrac{-3}{5}\right)\)
\(x\) \(=\left(\dfrac{-3}{5}\right)+\left(\dfrac{-1}{2}\right)=\dfrac{-11}{10}\)
\(\Rightarrow x\in\left\{\dfrac{1}{10};\left(\dfrac{-11}{10}\right)\right\}\)
\(\text{3)f}\left(\dfrac{-1}{2}\right)=\dfrac{1}{2}-3.\left(\dfrac{-1}{2}\right)=\dfrac{1}{2}-\left(\dfrac{-3}{2}\right)=2\)
\(f\left(\dfrac{1}{6}\right)=\dfrac{1}{2}-3.\dfrac{1}{6}=\dfrac{1}{2}-\dfrac{1}{2}=0\)
\(\text{b)Áp dụng tính chất dãy tỉ số bằng nhau:}\)
\(\dfrac{a}{2}=\dfrac{b}{3}=\dfrac{c}{4}=\dfrac{2a-b+3c}{2.3-3+3.4}=\dfrac{39}{13}=3\)
\(\Rightarrow a=3.2=6\)
\(b=3.3=9\)
\(c=3.4=12\)
Câu 4 mik ko biết làm,bn thông cảm:(((
