13.
\(a,\dfrac{4}{5}\times\dfrac{3}{13}-\dfrac{4}{5}\times\dfrac{16}{13}+\dfrac{4}{5}=\dfrac{4}{5}\times\left(\dfrac{3}{13}-\dfrac{16}{13}+1\right)=\dfrac{4}{5}\times0=0\)
\(b,7,12+\left(-18,02\right)+2,88=\left(7,12+2,88\right)-18,02=10-18,02=-8,02\)
14.
\(a,x-5,01=4,02\)
\(x=9,03\)
\(b,x-\dfrac{1}{4}=-\dfrac{21}{10}\)
\(x=-\dfrac{21}{10}+\dfrac{1}{4}\)
\(x=-\dfrac{37}{20}\)
