a)

\({{ - 15} \over {90}}=\frac{-15 \div 15}{90 \div 15}=\frac{-1}{6}\)

\({{120} \over {600}}=\frac{120 \div 120}{600 \div 120}=\frac{1}{5}\)

\({{ - 75} \over {150}}=\frac{-75 \div 75}{150 \div 75}=\frac{-1}{2}\) 

$BCNN(6;5;2)=30$

Quy đồng

\({{ - 15} \over {90}}=\frac{-1}{6}=\frac{-1.5}{6.5}=\frac{-5}{30}\)

\({{120} \over {600}}=\frac{1}{5}=\frac{1.6}{5.6}=\frac{6}{30}\)

\({{ - 75} \over {150}}=\frac{-1}{2}=\frac{-1.15}{2.15}=\frac{-15}{30}\) 

b)

\({{54} \over { - 90}}=\frac{-54 \div 18}{90 \div 18}=\frac{-3}{5}\)

\({{ - 180} \over {288}}=\frac{-180 \div 36}{288 \div 36}=\frac{-5}{8}\)

\({{60} \over { - 135}}=\frac{60 \div 15}{-135 \div 15}=\frac{4}{-9}=\frac{-4}{9}\)

$BCNN(5; 8; 9)=5.8.9=360$

Quy đồng

\({{54} \over { - 90}}=\frac{-3}{5}=\frac{-3.72}{5.72}=\frac{-216}{360}\)

\({{ - 180} \over {288}}=\frac{-5}{8}=\frac{-5.45}{8.45}=\frac{-225}{360}\)

\({{60} \over { - 135}}=\frac{-4}{9}=\frac{-4.40}{9.40}=\frac{-160}{360}\)