A=11.3+13.5+15.7+....+1(2n−1)(2n+1)11.3+13.5+15.7+....+1(2n−1)(2n+1)

=> 2A=21.3+23.5+25.7+......+2(2n−1)(2n+1)21.3+23.5+25.7+......+2(2n−1)(2n+1)

<=> 2A=1−13+13−15+17+.....+12n−2−12n+11−13+13−15+17+.....+12n−2−12n+1

<=>2A=1−12n+11−12n+1

<=> A=(1−12n+1)(1−12n+1).12.12

<=> A=12−12(2n+1)12−12(2n+1)

=>A<12n<12 (đpcm)