2 câu trả lời
12+56+1112+1920+2930+4142+5556
=1-11.2+1-12.3+1-13.4+1-14.5+1-15.6+1-16.7+1-17.8
=7-(11.2+12.3+..+17.8)
=7-(1-12+12-13+...
=7-(1-1/8)
=56/8-7/8
=49/8
Đáp án:
1/2+5/6+11/12+19/20+29/30+41/42+55/56=49/8
Giải thích các bước giải:
1/2+5/6+11/12+19/20+29/30+41/42+55/56
=1-1/2+1-1/6+1-1/12+1-1/20+1-1/30+1-1/42+1-1/56
=(1+1+1+1+1+1+1)-(1/2+1/6+1/12+1/20+1/30+1/42+1/56)
=1.7-(1/(1.2)+1/(2.3)+1/(3.4)+1/(4.5)+1/(5.6)+1/(6.7)+1/(7.8))
=7-(1/1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7+1/7-1/8)
=7-(1/1-1/8)
=7-(8/8-1/8)
=7-7/8
=56/8-7/8
=49/8
