Tình bằng cách thuận tiện nhất :
a) 1/10+2/20+3/30+4/40+5/50+6/60+7/70+8/80+9/90
=..................................................................................................................................................................................................
=....................................................................................................................................................................................................
b) 3,72 + 2,05 + 6,28 + 7,95 =..........................................................................................................................................................
=..........................................................................................................................................................
a, 1/10 + 2/20 + 3/30 + 4/40 + 5/50 + 6/60 + 7/70 + 8/80 + 9/90
= 1/10 + 1/10 + 1/10 + 1/10 + 1/10 + 1/10 + 1/10 + 1/10 + 1/10
= 1/10 x 9
= 9/10
b, 3.72 + 2.05 + 6.28 + 7.95
= (3.72 + 6.28) + (2.05 + 7.95)
= 10 + 10
= 20