41) 等于多?
(1/11 1/21 1/31 1/41)*(1/21 1/31 1/41 1/51)-(1/11 1/21 1/31 1/41 1/51)*(1/21 1/31 1/41) 等于多少
(1/11+1/21+1/31+1/41)*(1/21+1/31+1/41+1/51)-(1/11+1/21+1/31+1/41+/51)*(1/21+1/31+1/41) =【1/11+(1/21+1/31+1/41)】*(1/21+1/31+1/41+1/51)-【1/11+(1/21+1/31+1/41+/51)】*(1/21+1/31+1/41) =1/11*(1/21+1/31+1/41+1/51)+(1/21+1/31+1/41)(1/21+1/31+1/41+1/51)-1/11*(1/21+1/31+1/41)-(1/21+1/31+1/41)*(1/21+1/31+1/41+1/51) =1/11*(1/51)=1/561
答:1/12*13+1/13*14+1/14*15+....+1/19*20 =1/12-1/13+1/13-1/14+...+1/19-1/20 =1/12-1/...详情>>
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