行政能力测试
12+16+112+120+…+1n(n+1)(n为自然数)的值为() A.n+2n+1B.nn+1 C.n-1n+1D.n-2n+1
1/2+1/6+1/12+1/20+......+1/[n(n+1)] ??? =(2-1)/(1*2)+(3-2)/(2*3)+(4-3)/(3*4)+......+[(n+1)]/[n(n+1)] =(1-1/2)+(1/2-1/3)+(1/3-1/4)+......+[1/n-1/(n+1)] =1-1/(n+1) =n/(n+1) 故选 B.
12+16+112+120+…+1n(n+1) (n为自然数)的值为() A.n+2n+1 ,B.nn+1 ,C.n-1n+1 ,D.n-2n+1 这是选择题,用赋值法: n=1时,原式=12 ,A=32 ,B=12 ,C=2 ,D=? n=2时,原式=28 ,A=43,B=23 ,C=13 ,D=3 不知道该选什么,我当不了公务员啦。
答:根据列项相消有: =1/2-1/9=7/18详情>>
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