2*1/3+3*1/4+4*1/5+5*1/6+6*1/7+8*1/9+9*1/10
=1-1/3+1-1/4+1-1/5+1-1/6+1-1/7+1-/9+1-1/10
=7-(1/3+1/4+1/5+1/6+1/7+1/9+1/10)
=7-(1/3+1/4+1/6+1/7+1/9+1/5+1/10)
=7-(3/4+16/63+3/10)
=7-(21/20+16/63)
=7-(1+1/20+16/63)
=7-(1+(63+20*16/63*20))
=7-1-383/1260
=5又877/1260
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