(a-b)/(a+b)=(a+b-2b)/(a+b)
=1-2b/(a+b)
=1-2/(a/b+1)
=1-2/(2/3+1)
=-1/5
(x+1)/(y+1)+(y+1)/(x+1)
=(x²+2x+1+y²+2y+1)/(xy+x+y+1)
=[(x+y)²-2xy+2(x+y)+2]/(xy+x+y+1)
=(16+24-8+2)/(-12-4+1)
=34/(-15)
=-34/15
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