Ta có:
\(\dfrac{21}{3\cdot11}>\dfrac{12}{3\cdot11}\)
\(\dfrac{45}{11\cdot19}>\dfrac{12}{11\cdot19}\)
\(\dfrac{69}{19\cdot27}>\dfrac{12}{19\cdot27}\)
\(\Rightarrow\dfrac{21}{3\cdot11}-\dfrac{45}{11\cdot19}+\dfrac{69}{19\cdot27}>\dfrac{12}{3\cdot11}+\dfrac{12}{11\cdot19}+\dfrac{12}{19\cdot27}\)
\(\Rightarrow A>B\)
Vậy \(A>B\).