Giải thích các bước giải:
Ta có:
$B=\dfrac{3^2}{20.23}+\dfrac{3^2}{23.26}+\dfrac{3^2}{26.29}+...+\dfrac{3^2}{77.80}$
$\to B=3(\dfrac{3}{20.23}+\dfrac{3}{23.26}+\dfrac{3}{26.29}+...+\dfrac{3}{77.80})$
$\to B=3(\dfrac{23-20}{20.23}+\dfrac{26-23}{23.26}+\dfrac{29-26}{26.29}+...+\dfrac{80-77}{77.80})$
$\to B=3(\dfrac{1}{20}-\dfrac{1}{23}+\dfrac{1}{23}-\dfrac{1}{26}+\dfrac{1}{26}-\dfrac{1}{29}+...+\dfrac{1}{77}-\dfrac{1}{80})$
$\to B=3(\dfrac{1}{20}-\dfrac{1}{80})$
$\to B=3.\dfrac{3}{80}$
$\to B=\dfrac{9}{80}$
$\to B>\dfrac{9}{81}$
$\to B>\dfrac19$
