1/1*2+1/2*3+1/3*4+…+1/2009*2010
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1/1*2+1/2*3+1/3*4+…+1/2009*2010
1/1*2+1/2*3+1/3*4+…+1/2009*2010
1/1*2+1/2*3+1/3*4+…+1/2009*2010
用裂项相消法算
分母1 2 3 4.是以1为首相,公差为1的等差数列 相消时提公差的倒数
即:
1/1*2+1/2*3+1/3*4+…+1/2009*2010
=1/1(1-1/2+1/2-1/3.-1/2009+1/2010)
=1-1/2010
=2009/2010
这是数列常见的题型
分数拆分
1/1*2+1/2*3+1/3*4+…+1/2009*2010
=(1 - 1/2)+(1/2-1/3)+(1/3-1/4)+(1/4-1/5)+......+(1/2009-1/2010)
=1- 1/2010
=2009/2010
原式=(1-1/2)+(1/2-1/3)+(1/3-1/4)+(1/4-1/5)+…+(1/2008-1/2009)+(1/2009-1/2010)=1-1/2010=2009/2010
1/1*2+1/2*3+1/3*4+…+1/2009*2010
=(1-1/2)+(1/2-1/3)+(1/3-1/4)+...+(1/2009-1/2010)
=1-1/2+1/2-1/3+1/3-1/4+...+1/2009-1/2010
=1-1/2010
=2009/2010
原式 = 1×1/2 +1/2 ×1/3 + 1/3×1/4 +……+1/2009×1/2010
=(1-1/2)+(1/2-1/3)+(1/3-1/4)+……+(1/2009-1/2010)
=1-1/2+1/2-1/3+1/3-1/4+……+1/2009-1/2010
=1-1/2010
=2009/2010
(-1*1/2)+(-1/2*1/3)+(-1/3*1/4)+....+(1/2009*1/2010)
1/1*2+1/2*3+1/3*4+…+1/2009*2010
=(1-1/2)+(1/2-1/3)+(1/3-1/4)+...+(1/2009-1/2010)
=1-1/2+1/2-1/3+1/3-1/4+...+1/2009-1/2010
=1-1/2010
=2009/2010