计算1/[x(x+3)]+1/[(x+3)(x+6)]+......+1/[(x+15)(x+18)]=?
来源:百度知道 编辑:UC知道 时间:2024/05/10 12:31:42
我需要过程哦!
为什麽开始时是1/3??????????
为什麽开始时是1/3??????????
1/[x(x+3)]+1/[(x+3)(x+6)]+......+1/[(x+15)(x+18)]
=1/3[1/x-1/(x+3)]+[1/(x+3)-1/(x+6)]+......+[1/(x+15)-1/(x+18)]
=1/3[1/x-1/(x+3)+1/(x+3)-1/(x+6)+......+1/(x+15)-1/(x+18)]
=1/x-1/(x+18)
=1/3*(x+18-x)/x(x+18)
=6/(x^2+18x)
1/[x(x+3)]+1/[(x+3)(x+6)]+......+1/[(x+15)(x+18)]
={3/[x(x+3)]+3/[(x+3)(x+6)]+......+3/[(x+15)(x+18)]}/3
={[(1/x-1/(x+3)]+[1/(x+3)-1/(x+6)]+......+[1/(x+15)-1/(x+18)]}/3
=[1/x-1/(x+18)]/3
=[(x+18-x)/(x(x+18)]/3
=6/(x^2+18x)
用裂项法
原式=1/3*【1/x-1/(x+3)+1/(x+3)-1/(x+6)+...1/(x+15)-1/(x+18)】
中括号里的前后消掉,
原式=1/3*【1/x-1/(x+18)】
=-6/【x(x+18)]
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