y={(x-4)(x-3)/[(x-2)(x-1)]}^1/2的定义域是(-∞,1)∪(2,3]∪[4,+∞)
x≥4时,lny=1/2[ln(x-4)+ln(x-3)-ln(x-2)-ln(x-1)],
y′/y=1/2[1/(x-4)+1/(x-3)-1/(x-2)-1/(x-1)].(*)
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