简便计算:1/1×3+1/2×4+1/3×5+.+1/18×20
2个回答
1/1×3+1/2×4+1/3×5+.+1/18×20
=(1-1/3+1/2-1/4+1/3-1/5+.+1/18-1/20)/2
=(1+1/2-1/19-1/20)/2
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