weixiliu
幼苗
共回答了24个问题采纳率：95.8% 举报
（x+y)+(2x+1/1*2y)+(3x+1/2*3y)+.+(9x+1/8*9y)
= x(1+2+3+...9) + y { 1+1/(1*2)+1/(2*3)+1/(3*4)+...1/(8*9) }
= x(1+9)*9/2 + y { 1+1-1/2+1/2-1/3+1/3-1/4+...1/8-1/9) }
= 45x + y(2-1/9)
= 45x+2y-y/9
= 45*2 + 2*9 - 9/9
= 90+18-1
= 107
1年前
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