Chiều dài ban đầu của lò xo là: \(30-10=20\left(cm\right)\)

\(a,\dfrac{2}{3}.x=\dfrac{4}{7}\)

\(x=\dfrac{4}{7}:\dfrac{2}{3}\)

\(x=\dfrac{4}{7}.\dfrac{3}{2}\)

\(x=\dfrac{12}{14}=\dfrac{6}{7}\)

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\(b,\dfrac{-6}{5}.x=\dfrac{1}{2}\)

\(x=\dfrac{1}{2}:\dfrac{-6}{5}\)

\(x=\dfrac{1}{2}.\dfrac{-5}{6}\)

\(x=\dfrac{-5}{12}\)

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\(c,\dfrac{2}{3}.x=\dfrac{-1}{4}\)

\(x=\dfrac{-1}{4}:\dfrac{2}{3}\)

\(x=\dfrac{-1}{4}.\dfrac{3}{2}\)

\(x=\dfrac{-3}{8}\)

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\(d,\dfrac{3}{4}.x=\dfrac{2}{-7}\)

\(x=\dfrac{2}{-7}:\dfrac{3}{4}\)

\(x=\dfrac{2}{-7}.\dfrac{4}{3}\)

\(x=\dfrac{-8}{21}\)

Tống diện tích người đó cần sơn là: \(15\times15\times5=1125\left(cm^2\right)\)

\(\dfrac{19\times2\times5}{19\times3\times5}=\dfrac{2}{3}\)

\(\left(y+10,3\right)\times10,8=270\)

\(y+10,3=270:10,8\)

\(y+10,3=25\)

\(y=25-10,3\)

\(y=14,7\)

(Cho mik làm lại nha)

\(a,\dfrac{1}{2}+\left(-\dfrac{3}{4}\right)=\dfrac{2}{4}+\left(-\dfrac{3}{4}\right)=-\dfrac{1}{4}\)

\(a,\dfrac{2}{5}+\left(-\dfrac{1}{16}\right)+\left(-\dfrac{9}{5}\right)+\left(-\dfrac{5}{16}\right)=\left(\dfrac{2}{5}+\dfrac{-9}{5}\right)-\left(\dfrac{1}{16}+\dfrac{5}{16}\right)=\dfrac{-7}{5}-\dfrac{6}{16}=\dfrac{-71}{40}\)

\(c,\dfrac{-2}{13}+\dfrac{8}{15}+\dfrac{10}{20}+\dfrac{15}{13}+\left(-\dfrac{23}{15}\right)=\left(\dfrac{-2}{13}+\dfrac{15}{13}\right)+\left(\dfrac{8}{15}+\dfrac{-23}{15}\right)+\dfrac{1}{2}=1+\left(-1\right)+\dfrac{1}{2}=\dfrac{1}{2}\)

\(a,\dfrac{1}{2}+\left(-\dfrac{3}{4}\right)=\dfrac{2}{4}+\left(-\dfrac{3}{4}\right)=-\dfrac{1}{4}\)

\(b,\dfrac{2}{5}+\left(-\dfrac{1}{16}\right)+\left(-\dfrac{9}{5}\right)+\left(-\dfrac{5}{16}\right)=\left(\dfrac{2}{5}-\dfrac{9}{5}\right)-\left(\dfrac{1}{16}+\dfrac{5}{16}\right)=\dfrac{-7}{5}-1=\dfrac{-12}{5}\)

\(c,\dfrac{-2}{13}+\dfrac{8}{15}+\dfrac{10}{20}+\dfrac{15}{13}+\left(-\dfrac{23}{15}\right)=\left(\dfrac{-2}{13}+\dfrac{15}{13}\right)+\left(\dfrac{8}{15}+\dfrac{-23}{15}\right)+\dfrac{1}{2}=1+\left(-1\right)+\dfrac{1}{2}=\dfrac{1}{2}\)