每天过来问候你!泰勒你好!
1、泰勒(Taylor)公式
泰勒公式1:
如果函数 f ( x ) f(x) f(x) 在 x 0 x_{0} x0 处具有 n n n 阶导数,那么存在 x 0 x_{0} x0 的一个邻域,对于该邻域内的任一 x x x,有:
f ( x ) = f ( x 0 ) + f ′ ( x 0 ) ( x − x 0 ) + f ′ ′ ( x 0 ) 2 ! ( x − x 0 ) 2 + ⋯ + f ( n ) ( x 0 ) n ! ( x − x 0 ) n + o ( ( x − x 0 ) n ) f(x)=f(x_{0})+f^{\prime}(x_{0})(x-x_{0})+\dfrac{f^{\prime\prime}(x_{0})}{2!}(x-x_{0})^{2}+\cdots +\dfrac{f^{(n)}(x_{0})}{n!}(x-x_{0})^{n}+o\Big((x-x_{0})^{n}\Big) f(x)=f(x0)+f′(x0)(x−x0)+2!f′′(x0)(x−x0)2+⋯+n!f(n)(x0)(x−x0)n+o((x−x0)n)。
佩亚诺型余项
注:泰勒公式的唯一性:
如果函数 f ( x ) f(x) f(x) 在 x 0 x_{0} x0 处具有 n n n 阶导数,且 f ( x ) = a 0 + a 1 ( x − x 0 ) + a 2 ( x − x 0 ) 2 + ⋯ + a n ( x − x 0 ) n + o ( ( x − x 0 ) n ) f(x)=a_{0}+a_{1}(x-x_{0})+a_{2}(x-x_{0})^{2}+\cdots +a_{n}(x-x_{0})^{n}+o\Big((x-x_{0})^{n}\Big) f(x)=a0+a1(x−x0)+a2(x−x0)2+⋯+an(x−x0)n+o((x−x0)n),
则 a 0 = f ( x 0 ) , a 1 = f ′ ( x 0 ) , a 2 = f ′ ′ ( x 0 ) 2 ! , ⋯ , a n = f ( n ) ( x 0 ) n ! a_{0}=f(x_{0}),a_{1}=f^{\prime}(x_{0}),a_{2}=\dfrac{f^{\prime\prime}(x_{0})}{2!},\cdots ,a_{n}=\dfrac{f^{(n)}(x_{0})}{n!} a0=f(x0),a1=f′(x0),a2=2!f′′(x0),⋯,an=n!f(n)(x0)。
泰勒公式2:
如果函数 f ( x ) f(x) f(x) 在 x 0 x_{0} x0 的一个邻域内具有 ( n + 1 ) (n+1) (n+1) 阶导数,那么对于该邻域内的任一 x x x,有:
f ( x ) = f ( x 0 ) + f ′ ( x 0 ) ( x − x 0 ) + f ′ ′ ( x 0 ) 2 ! ( x − x 0 ) 2 + ⋯ + f ( n ) ( x 0 ) n ! ( x − x 0 ) n + f ( n + 1 ) ( ξ ) ( n + 1 ) ! ( x − x 0 ) n + 1 f(x)=f(x_{0})+f^{\prime}(x_{0})(x-x_{0})+\dfrac{f^{\prime\prime}(x_{0})}{2!}(x-x_{0})^{2}+\cdots +\dfrac{f^{(n)}(x_{0})}{n!}(x-x_{0})^{n}+\dfrac{f^{(n+1)}(\xi)}{(n+1)!}(x-x_{0})^{n+1} f(x)=f(x0)+f′(x0)(x−x0)+2!f′′(x0)(x−x0)2+⋯+n!f(n)(x0)(x−x0)n+(n+1)!f(n+1)(ξ)(x−x0)n+1,
其中 ξ \xi ξ 介于 x 0 x_{0} x0 与 x x x 之间。
拉格朗日型余项
注: 若取 x 0 = 0 x_{0}=0 x0=0,则上述泰勒公式称为麦克劳林公式。
2、重要的麦克劳林公式
1 1 − x = ∑ n = 0 ∞ = 1 + x + x 2 + x 3 + ⋯ + x n + o ( x n ) ; x ∈ ( − 1 , 1 ) \displaystyle\large{\dfrac{1}{1-x}= \sum_{n=0}^{\infty} =1+x+x^{2}+x^{3}+\cdots +x^{n}+o(x^{n})};x \in (-1,1) 1−x1=n=0∑∞=1+x+x2+x3+⋯+xn+o(xn);x∈(−1,1)
e x = ∑ n = 0 ∞ = 1 + x + 1 2 ! x 2 + 1 3 ! x 3 + ⋯ + 1 n ! x n + o ( x n ) ; x ∈ ( − ∞ , + ∞ ) \displaystyle\large{e^{x}= \sum_{n=0}^{\infty} =1+x+\dfrac{1}{2!}x^{2}+\dfrac{1}{3!}{x^{3}}+\cdots +\dfrac{1}{n!}{x^{n}}+o(x^{n})};x \in (-\infty,+\infty) ex=n=0∑∞=1+x+2!1x2+3!1x3+⋯+n!1xn+o(xn);x∈(−∞,+∞)
s i n x = ∑ n = 0 ∞ = x − 1 3 ! x 3 + 1 5 ! x 5 − 1 7 ! x 7 + ⋯ + ( − 1 ) n ( 2 n + 1 ) ! x 2 n + 1 + o ( x 2 n + 2 ) ; x ∈ ( − ∞ , + ∞ ) \displaystyle\large{sinx= \sum_{n=0}^{\infty} =x-\dfrac{1}{3!}{x^{3}}+\dfrac{1}{5!}{x^{5}}-\dfrac{1}{7!}{x^{7}}+\cdots +\dfrac{(-1)^n}{(2n+1)!}{x^{2n+1}}+o(x^{2n+2})};x \in (-\infty,+\infty) sinx=n=0∑∞=x−3!1x3+5!1x5−7!1x7+⋯+(2n+1)!(−1)nx2n+1+o(x2n+2);x∈(−∞,+∞)
c o s x = ∑ n = 0 ∞ = 1 − 1 2 ! x 2 + 1 4 ! x 4 − 1 6 ! x 6 + ⋯ + ( − 1 ) n ( 2 n ) ! x 2 n + o ( x 2 n + 1 ) ; x ∈ ( − ∞ , + ∞ ) \displaystyle\large{cosx= \sum_{n=0}^{\infty} =1-\dfrac{1}{2!}{x^{2}}+\dfrac{1}{4!}{x^{4}}-\dfrac{1}{6!}{x^{6}}+\cdots +\dfrac{(-1)^{n}}{(2n)!}{x^{2n}}+o(x^{2n+1})};x \in (-\infty,+\infty) cosx=n=0∑∞=1−2!1x2+4!1x4−6!1x6+⋯+(2n)!(−1)nx2n+o(x2n+1);x∈(−∞,+∞)
l n ( 1 + x ) = ∑ n = 1 ∞ = x − 1 2 x 2 + 1 3 x 3 − 1 4 x 4 + ⋯ + ( − 1 ) n − 1 n x n + o ( x n ) ; x ∈ ( − 1 , 1 ] \displaystyle\large{ln(1+x)= \sum_{n=1}^{\infty} =x-\dfrac{1}{2}{x^{2}}+\dfrac{1}{3}{x^{3}}-\dfrac{1}{4}{x^{4}}+\cdots +\dfrac{(-1)^{n-1}}{n}{x^{n}}+o(x^n)};x \in (-1,1] ln(1+x)=n=1∑∞=x−21x2+31x3−41x4+⋯+n(−1)n−1xn+o(xn);x∈(−1,1]
( 1 + x ) a = 想 一 想 = 1 + a x + a ( a − 1 ) 2 ! x 2 + ⋯ + a ( a − 1 ) ⋯ ( a − n + 1 ) n ! x n + o ( x n ) \displaystyle\large{(1+x)^a= 想一想 =1+ax+\dfrac{a(a-1)}{2!}{x^{2}}+\cdots +\dfrac{a(a-1)\cdots(a-n+1)}{n!}{x^{n}}+o(x^n)} (1+x)a=想一想=1+ax+2!a(a−1)x2+⋯+n!a(a−1)⋯(a−n+1)xn+o(xn)
3、无穷小等价问题 o ( x n ) o(x^{n}) o(xn)
o ( x 2 ) ± o ( x 2 ) = o ( x 2 ) o(x^2) \pm o(x^2)=o(x^2) o(x2)±o(x2)=o(x2)
o ( x 2 ) ± o ( x 3 ) = o ( x 2 ) o(x^2) \pm o(x^3)=o(x^2) o(x2)±o(x3)=o(x2)
x 2 × o ( x 3 ) = o ( x 5 ) \ \ x^2 \ \ \ \times o(x^3)=o(x^5) x2 ×o(x3)=o(x5)
o ( x 2 ) × o ( x 3 ) = o ( x 5 ) o(x^2) \times o(x^3)=o(x^5) o(x2)×o(x3)=o(x5)
x 3 = o ( x 2 ) \ \ \ x^3\qquad \qquad =o(x^2) x3=o(x2)
o ( x 3 ) = o ( x 2 ) \ o(x^3)\qquad \ \quad =o(x^2) o(x3) =o(x2)
注:只能从左向右换算;
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