f(x)=x-2/x+a(2-Inx) --> x>0
f'(x)=1+2/x^2-a/x > 0 -->
f'(x)=x^2 -ax +2 > 0
1）當 Δ=a^2-8 ≤ 0 即：0 0)
當 [a-√(a^2-8)]/2 < x < [a+√(a^2-8)]/2 函數(shù)單調(diào)遞減,
當 x[a+√(a^2-8)]/2 函數(shù)單調(diào)遞增.