12+16+112+120+…+1n(n+1)(n為自然數(shù))的值為()　　A.n+2n+1B.nn+1　　C.n-1n+1D.n-2n+1

熱心網(wǎng)友

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)故選 B.

熱心網(wǎng)友

12+16+112+120+…+1n(n+1)　　(n為自然數(shù))的值為()　　A.n+2n+1　，B.nn+1　，C.n-1n+1　　，D.n-2n+1　這是選擇題，用賦值法：n=1時，原式＝12 ,A=32 ,B=12 ,C=2 ,D=？n=2時，原式＝28 ,A=43,B=23 ,C=13 ,D=3不知道該選什么，我當不了公務(wù)員啦。