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(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)=?
人气:296 ℃ 时间:2019-08-20 11:08:18
解答
设1/21+1/31+1/41=A,原式 =1/11A+A^2+1/51A+1/11*1/51-1/11A-1/51A-A^2=1/11*1/51=1/561
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