1:we _________his uncle's yesterday.A:called on B:arrived to C:reached at D:called at2:Quite a number of people think ______ necessary to learn Chinese history.A.it B.that is C.this 都选哪个?为什么呢?

1:we _________his uncle's yesterday.A:called on B:arrived to C:reached at D:called at2:Quite a number of people think ______ necessary to learn Chinese history.A.it B.that is C.this 都选哪个?为什么呢? Jim,my friend,_________his brother. UN 如果级数Un收敛,1/Un的敛散性? {Un}收敛于a,{Un+1}收敛于多少 I hope we should be friend and come to un... 若lim Un=A>0,用数列定义证明lim Un+1 / Un =1 极限un 等于a则级数（un-un-1）收敛于什么? 设数列un收敛于S,则级数un+1-un收敛于 证明：若正项级数∑Un收敛,则∑Un/(1+Un)也收敛 若级数Un收敛于s 则级数(un+un+1)收敛于 设级数∑un收敛,证明∑(un+un+1)也收敛 级数un收敛 那么级数un^2-un+1^2收敛吗 Tom _________his father and his fatherTom _________his father and his father_________very young 设数列un由下式定义u1=2,un+1=un(un^2+3)/(3un^2+1)(n=1,2……)试证数列un收敛 当满足条件（ n=1∑Un 收敛 ）时,n=1∑(-1)^n Un (Un>0) 若级数∑Un收敛于S,级数∑【un+un+1】则收敛于{n从1到无穷｝ 一个交错级数的问题,莱布尼茨定理其中一个条件是满足条件Un>=Un+1 ,那如果Un