7×8/15-7×12/35+7×16/63-7×20/99
=7×（8/15-12/35+16/63-20/99）
=7×（1/3+1/5-1/5-1/7+1/7+1/9-1/9-1/11）
=7×（1/3-1/11）
=7×8/33
=56/33
=1又23/33
=7x(8/15-12/35十16/63-22/99)=
7*(8/15-12/35+16/63-2/9)=7*(8/15-12/35+2/63)
=7*8/15-12/5+2/9
=56/15-36/15+z/9
=2又2/9
8/15*7-7*12/35+7*16/63-7*22/99
=7*(1/3+1/5-1/5-1/7+1/7-1/9+2/9)
=1/7*4/9
=4/63
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