Question

-1/20+ -1/30+ -1/42+ -1/56+ -1/72+ -1/90

Answer 1

đặt dấu âm ra ngoài

=-(1/20+1/30+1/42+..........1/90)

=-(1/4.5+1/5.6+1/6.7+......1/9.10)

=-(1/4-1/5+1/5-1/6+.....+1/9-1/10)

=-(1/4-1/10)

=-0,15