Let f(x)=1+x/1!+x^2/2!+x^3/3!+x^4/4!. The number of real roots of f ( x ) = 0 is

0 ⇒ f'(x) is an increasing \f^\n ⇒ f'(x) = 0 at \x = \x_0 \Rightarrow\f'(x_0)=0 \Rightarrow1+x_0+x^2_0/2+x_0^3/6=0.... (1) f'(-2) f'(-1) 0" >

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