f(x)=-x^3+ax^2-4
f(1)f'(-1)=-8
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解ï¼f(1)=-1+a-4=a-5
f'=-3x^2+2ax
f'(1)=-3x1^2+2a=-3+2a=2a-3
(a-5)(2a-3)=-8
2a^2-3a-10a+15+8=0
2a^2-13a+23=0
169-4x2x23=169-184=-15<0
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