解一题：
-x²+x-3=-(x²-x+3)
x²-x+3与-x²+x-3的最简公分母是(x²-x+3)
x平方-x+3分之6x+1=(6x+1)/(x²-x+3)
-x平方+x-3分之4x-3=-(4x-3)/(x²-x+3)
解二题：
(x-1)(2-x)=-(x-1)(x-2)
(x-1)(x-2)与(x-1)(2-x)的最简公分母是(x-1)(x-2)
(x-1)(x-2)分之3x=3x/[(x-1)(x-2)]
(x-1)(2-x)分之3+x=-(3+x)/[(x-1)(x-2)]