按规律写下面各题：（1）2乘以3分之1加3乘以4分之1加4乘以5分之1等于多少；（2）3乘以4分之

按规律写下面各题：（1）2乘以3分之1加3乘以4分之1加4乘以5分之1等于多少；（2）3乘以4分之1加4乘以5分之1加5乘以6分之1加6乘以7分之1等于多少

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⑴1/2×3+1/3×4+1/4×5
=(1/2-1/3)+(1/3-1/4)+(1/4-1/5)
=1/2-1/5
=3/10
⑵1/3×4+1/4×5+1/5×6+1/6×7
=(1/3-1/4)+(1/卜迅春4-1/5)+(1/5-1/6)+(1/54-1/型耐7)
=1/3-1/7
=4/21。昌启

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