\(\dfrac{4}{5}\times\dfrac{3}{8}=\dfrac{1}{5}\times\dfrac{3}{2}=\dfrac{3}{10}\)

B

C

\(a,58\times75+58\times50-58\times25=58\times\left(75+50-25\right)=58\times100=5800\)

\(b,91\times25-91\times13-91\times12=91\times\left(25-13-12\right)=91\times0=0\)

\(c,25\times36-25\times32+25=25\times\left(36-32+1\right)=25\times5=125\)

\(d,121\times74+74\times45-66\times74=74\times\left(121+45-66\right)=74\times100=7400\)\(e,136\times23+27\times136-136\times40=136\times\left(23+27-40\right)=136\times10=1360\)

\(7^2\times7\times\left(13-x\right)=14\)

\(\Leftrightarrow343\times\left(13-x\right)=14\)

\(\Leftrightarrow13-x=\dfrac{2}{49}\)

\(\Leftrightarrow x=13-\dfrac{2}{49}\)

\(\Leftrightarrow x=\dfrac{637}{49}-\dfrac{2}{49}\)

\(\Leftrightarrow x=\dfrac{635}{49}\)

\(-20nC=-2\times10^{-8}\left(C\right)\)

\(40nC=4\times10^{-8}\left(C\right)\)

                                      Giải

\(E=K\dfrac{\left|q_1\times q_2\right|}{r^2}=9\times10^{-9}\times\dfrac{\left|-2\times10^{-8}\times4\times10^{-8}\right|}{r^2}\)

\(\Rightarrow r=1.2\left(m\right)\)

\(7\dfrac{8}{10}=\dfrac{78}{10}=7.8\)

\(52\dfrac{65}{100}=52\dfrac{13}{20}=\dfrac{1053}{20}=52.65\)

\(815\dfrac{229}{1000}=815.299\)

\(58\times\left(x+56\right)=4176\)

\(\Leftrightarrow x+56=\dfrac{4176}{58}\)

\(\Leftrightarrow x+56=72\)

\(\Leftrightarrow x=72-56\)

\(x=16\)