求幂级数∑x^(2n)/(2n)!的和函数

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求幂级数∑x^(2n)/(2n)!的和函数求幂级数∑x^(2n)/(2n)!的和函数求幂级数∑x^(2n)/(2n)!的和函数considerTaylorexpansionofe^xe^x=1+x+x

求幂级数∑x^(2n)/(2n)!的和函数
求幂级数∑x^(2n)/(2n)!的和函数

求幂级数∑x^(2n)/(2n)!的和函数
consider Taylor expansion of e^x
e^x = 1 + x + x^2/2!+ ...+ x^n/n!+.(1)
e^(-x) = 1-x+x^2/2!+..;...+ (2)
(1)+(2)
e^(x) +e^(-x) = 2(1+x^2/2!+x^4/4!+...) = 2∑x^(2n)/(2n)!
∑x^(2n)/(2n)!=(1/2)(e^(x) +e^(-x))