1.x+2/x=3,x²+2-3x=0,x²-3x+2=0,(x-1)(x-2)=0,x1=1,x2=2；
2.x+6/x=5,x²+6-5x=0,x²-5x+6=0,(x-2)(x-3)=0,x1=2,x2=3；
3.x+12/x=7,x²+12-7x=0,x²-7x+12=0,(x-3)(x-4)=0,x1=3,x2=4；
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x+n(n+1)/x=2n+1,x²+n(n+1)-(2n+1)x=0,x²-(2n+1)x+n(n+1)=0,(x-n)(x-n-1)=0,x1=n,x2=n+1.