Giải thích các bước giải:
Ta có :
$A=\dfrac{3^2}{20.23}+\dfrac{3^2}{23.26}+...+\dfrac{3^2}{77.80}$
$\to A=3(\dfrac{3}{20.23}+\dfrac{3}{23.26}+...+\dfrac{3}{77.80})$
$\to A=3(\dfrac{23-20}{20.23}+\dfrac{26-23}{23.26}+...+\dfrac{80-77}{77.80})$
$\to A=3(\dfrac1{20}-\dfrac1{23}+\dfrac1{23}-\dfrac1{26}+...+\dfrac1{77}-\dfrac1{80})$
$\to A=3(\dfrac1{20}-\dfrac{1}{80})$
$\to A<3\cdot\dfrac1{20}$
$\to A<3\cdot\dfrac13$
$\to A<1$
