$A$ = $\frac{15}{1.6}$ + $\frac{15}{6.11}$ + $\frac{15}{11.16}$ + $\frac{15}{16.21}$ + $\frac{15}{21.26}$ + $\frac{15}{26.31}$ + $\frac{15}{31.36}$
=3.( $\frac{5}{1.6}$ + $\frac{5}{6.11}$ + $\frac{5}{11.16}$ + $\frac{5}{16.21}$ + $\frac{5}{21.26}$ + $\frac{5}{26.31}$ + $\frac{5}{31.36}$ )
=3.( 1 - $\frac{1}{6}$ + $\frac{1}{6}$ - $\frac{1}{11}$ + $\frac{1}{11}$ - $\frac{1}{16}$ + $\frac{1}{16}$ - $\frac{1}{21}$ + $\frac{1}{21}$ - $\frac{1}{26}$ + $\frac{1}{26}$ - $\frac{1}{31}$ + $\frac{1}{31}$ - $\frac{1}{36}$)
=3.(1-$\frac{1}{36}$)
=3.$\frac{35}{36}$
=$\frac{35}{12}$
Ta có : 2=$\frac{24}{12}$
$A$=$\frac{35}{12}$
⇒$A$>2(do $\frac{35}{12}$ >$\frac{24}{12}$)
