令y=[1+(a/x)]^x
两边同时取自然对数,得:
㏑y=㏑{[1+(a/x)]^x}
即㏑y=x㏑[1+(a/x)]
lim(x→∞)x㏑[1+(a/x)]
=lim(x→∞){㏑[1+(a/x)]}/(1/x)
根据洛必达法则:
lim(x→∞){㏑[1+(a/x)]}/(1/x)
=lim(x→∞){(-a/x²)[x/(x+a)]}/(-1/x²)
=lim(x→∞)ax²/[x(x+1)]
=lim(x→∞)2ax/2x+a
=2a/2
=a
∴lim(x→∞)[1+(a/x)]^x=e^a
至于lim(x→∞)[1+(1/x)]^x=e的证明,把a换成1就行了