设f(x)在x=0的邻域内有三阶导数,且x->0时,lim(1+x+f(x)/x)^(1/x)=e^3.求(1):f(0
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设f(x)在x=0的邻域内有三阶导数,且x->0时,lim(1+x+f(x)/x)^(1/x)=e^3.求(1):f(0),f'(0).f''(0)
求(2)lim(1+f(x)/x)^(1/x).
求(2)lim(1+f(x)/x)^(1/x).
1、条件等价于ln(1+x+f(x)/x)/x的极限是3.显然f(0)=0.另外,由于分母x趋于0,因此分子ln函数必趋于0,于是得x+f(x)/x趋于0,f(x)/x趋于0,即[f(x)-f(0)]/(x-0)趋于0,于是f'(0)=0.
利用Taylor展式得f(x)/x=f''(0)x^2/2+小o(x^2),因此
ln(1+x+f(x)/x^2)等价于x+f''(0)x/2,故极限值为1+f''(0)/2=3,解得f''(0)=4.
2、lim ln(1+f(x)/x)/x=lim ln(1+f''(0)x/2+小o(x))/x=lim 【f''(0)x/2+小o(x)】/x=f''(0)/2=2.
因此原极限为e^2
利用Taylor展式得f(x)/x=f''(0)x^2/2+小o(x^2),因此
ln(1+x+f(x)/x^2)等价于x+f''(0)x/2,故极限值为1+f''(0)/2=3,解得f''(0)=4.
2、lim ln(1+f(x)/x)/x=lim ln(1+f''(0)x/2+小o(x))/x=lim 【f''(0)x/2+小o(x)】/x=f''(0)/2=2.
因此原极限为e^2
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