1/30+1/42+1/56+1/72 简算

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1/30+1/42+1/56+1/72 简算

1/30+1/42+1/56+1/72 简算
1/30+1/42+1/56+1/72 简算

1/30+1/42+1/56+1/72 简算
答案是4/45,把1/30=1/5-1/6,同理,这样互相抵消,就剩1/5-1/9=4/45.

=1/5*6+1/6*7+1/7*8+1/8*9
=1/5-1/6+1/6-1/7+1/7-1/8+1/8-1/9
=1/5-1/9
=4/45

1/30=1/(5*6)
原式=1/5-1/6+1/6-1/7+1/7-1/8+1/8-1/9=1/5-1/9=4/45

=1/5*6+1/6*7+1/7*8+1/8*9
=1/5-1/6+1/6-1/7+1/7-1/8+1/8-1/9
=1/5-1/9
=4/45