高等数学(第七版)同济大学 习题12-8 个人解答
高等数学(第七版)同济大学 习题12-8
1. 将下列各周期函数展开成傅里叶级数(下面给出函数在一个周期内的表达式): \begin{aligned}&1. \ 将下列各周期函数展开成傅里叶级数(下面给出函数在一个周期内的表达式):&\end{aligned} 1. 将下列各周期函数展开成傅里叶级数(下面给出函数在一个周期内的表达式):
( 1 ) f ( x ) = 1 − x 2 ( − 1 2 ≤ x < 1 2 ) ; ( 2 ) f ( x ) = { x , − 1 ≤ x < 0 , 1 , 0 ≤ x < 1 2 , − 1 , 1 2 ≤ x < 1 ; ( 3 ) f ( x ) = { 2 x + 1 , − 3 ≤ x < 0 , 1 , 0 ≤ x < 3. \begin{aligned} &\ \ (1)\ \ f(x)=1-x^2\ \left(-\frac{1}{2} \le x \lt \frac{1}{2}\right);\\\\ &\ \ (2)\ \ f(x)=\begin{cases}x,-1 \le x \lt 0,\\\\1,0 \le x \lt \frac{1}{2},\\\\-1,\frac{1}{2}\le x \lt 1;\end{cases}\\\\ &\ \ (3)\ \ f(x)=\begin{cases}2x+1,-3 \le x \lt 0,\\\\1,0 \le x \lt 3.\end{cases} & \end{aligned} (1) f(x)=1−x2 (−21≤x<21); (2) f(x)=⎩ ⎨ ⎧x,−1≤x<0,1,0≤x<21,−1,21≤x<1; (3) f(x)=⎩ ⎨ ⎧2x+1,−3≤x<0,1,0≤x<3.
解:
( 1 ) 函数 f ( x ) 是半周期 l = 1 2 的偶函数,所以 b n = 0 ( n = 1 , 2 , ⋅ ⋅ ⋅ ) , a 0 = 4 ∫ 0 1 2 ( 1 − x 2 ) d x = 11 6 , a n = 4 ∫ 0 1 2 ( 1 − x 2 ) c o s ( 2 n π x ) d x = 4 [ 1 − x 2 2 n π s i n ( 2 n π x ) − 2 x 4 n 2 π 2 c o s ( 2 n π x ) + 2 8 n 3 π 3 s i n ( 2 n π x ) ] 0 1 2 = ( − 1 ) n + 1 n 2 π 2 ( n = 1 , 2 , ⋅ ⋅ ⋅ ) ,因为 f ( x ) 满足收敛定理的条件且处处连续, 所以 f ( x ) = 11 12 + 1 π 2 ∑ n = 1 ∞ ( − 1 ) n + 1 n 2 c o s ( 2 n π x ) , x ∈ ( − ∞ , + ∞ ) . ( 2 ) 函数 f ( x ) 的半周期为 l = 1 , a 0 = ∫ − 1 1 f ( x ) d x = ∫ − 1 0 x d x + ∫ 0 1 2 d x + ∫ 1 2 1 ( − 1 ) d x = − 1 2 , a n = ∫ − 1 1 f ( x ) c o s ( n π x ) d x = ∫ − 1 0 x c o s ( n π x ) d x + ∫ 0 1 2 c o s ( n π x ) d x − ∫ 1 2 1 c o s ( n π x ) d x = [ x n π s i n ( n π x ) + 1 n 2 π 2 c o s ( n π x ) ] − 1 0 + [ 1 n π s i n ( n π x ) ] 0 1 2 + [ 1 n π s i n ( n π x ) ] 1 1 2 = 1 n 2 π 2 [ 1 − ( − 1 ) n ] + 2 n π s i n n π 2 ( n = 1 , 2 , ⋅ ⋅ ⋅ ) , b n = ∫ − 1 1 f ( x ) s i n ( n π x ) d x = ∫ − 1 0 x s i n ( n π x ) d x + ∫ 0 1 2 s i n ( n π x ) d x − ∫ 1 2 1 s i n ( n π x ) d x = − 2 n π c o s n π 2 + 1 n π ( n = 1 , 2 , ⋅ ⋅ ⋅ ) ,因为 f ( x ) 满足收敛定理的条件,其间断点 x = 2 k , 2 k + 1 2 , k ∈ Z , 所以 f ( x ) = − 1 4 + ∑ n = 1 ∞ { [ 1 − ( − 1 ) n n 2 π 2 + 2 n π s i n n π 2 ] c o s ( n π x ) + 1 n π ( 1 − 2 c o s n π 2 ) s i n ( n π x ) } , x ∈ R , { 2 k , 2 k + 1 2 ∣ k ∈ Z } ( 3 ) 函数 f ( x ) 的半周期 l = 3 , a 0 = 1 3 ∫ − 3 3 f ( x ) d x = 1 3 [ ∫ − 3 0 ( 2 x + 1 ) d x + ∫ 0 3 d x ] = − 1 , a n = 1 3 ∫ − 3 3 f ( x ) c o s n π x 3 d x = 1 3 [ ∫ − 3 0 ( 2 x + 1 ) c o s n π x 3 d x + ∫ 0 3 c o s n π x 3 d x ] = 6 n 2 π 2 [ 1 − ( − 1 ) n ] ( n = 1 , 2 , ⋅ ⋅ ⋅ ) , b n = 1 3 ∫ − 3 3 f ( x ) s i n n π x 3 d x = 1 3 [ ∫ − 3 0 ( 2 x + 1 ) s i n n π x 3 d x + ∫ 0 3 s i n n π x 3 d x ] = 6 n π ( − 1 ) n + 1 ( n = 1 , 2 , ⋅ ⋅ ⋅ ) , 因为 f ( x ) 满足收敛定理的条件,其间断点为 x = 3 ( 2 k + 1 ) , k ∈ Z , 所以 f ( x ) = − 1 2 + ∑ n = 1 ∞ { 6 n 2 π 2 [ 1 − ( − 1 ) n ] c o s n π x 3 + ( − 1 ) n + 1 6 n π s i n n π x 3 } , x ∈ R , { 3 ( 2 k + 1 ) ∣ k ∈ Z } . \begin{aligned} &\ \ (1)\ 函数f(x)是半周期l=\frac{1}{2}的偶函数,所以b_n=0\ (n=1,2,\cdot\cdot\cdot),a_0=4\int_{0}^{\frac{1}{2}}(1-x^2)dx=\frac{11}{6},\\\\ &\ \ \ \ \ \ \ \ a_n=4\int_{0}^{\frac{1}{2}}(1-x^2)cos(2n\pi x)dx=4\left[\frac{1-x^2}{2n\pi}sin(2n\pi x)-\frac{2x}{4n^2\pi^2}cos(2n\pi x)+\frac{2}{8n^3\pi^3}sin(2n\pi x)\right]_{0}^{\frac{1}{2}}=\\\\ &\ \ \ \ \ \ \ \ \frac{(-1)^{n+1}}{n^2\pi^2}\ (n=1,2,\cdot\cdot\cdot),因为f(x)满足收敛定理的条件且处处连续,\\\\ &\ \ \ \ \ \ \ \ 所以f(x)=\frac{11}{12}+\frac{1}{\pi^2}\sum_{n=1}^{\infty}\frac{(-1)^{n+1}}{n^2}cos(2n\pi x),x \in (-\infty, \ +\infty).\\\\ &\ \ (2)\ 函数f(x)的半周期为l=1,a_0=\int_{-1}^{1}f(x)dx=\int_{-1}^{0}xdx+\int_{0}^{\frac{1}{2}}dx+\int_{\frac{1}{2}}^{1}(-1)dx=-\frac{1}{2},\\\\ &\ \ \ \ \ \ \ \ a_n=\int_{-1}^{1}f(x)cos(n\pi x)dx=\int_{-1}^{0}xcos(n\pi x)dx+\int_{0}^{\frac{1}{2}}cos(n\pi x)dx-\int_{\frac{1}{2}}^{1}cos(n\pi x)dx=\\\\ &\ \ \ \ \ \ \ \ \left[\frac{x}{n\pi}sin(n\pi x)+\frac{1}{n^2\pi^2}cos(n\pi x)\right]_{-1}^{0}+\left[\frac{1}{n\pi}sin(n\pi x)\right]_{0}^{\frac{1}{2}}+\left[\frac{1}{n\pi}sin(n\pi x)\right]_{1}^{\frac{1}{2}}=\\\\ &\ \ \ \ \ \ \ \ \frac{1}{n^2\pi^2}[1-(-1)^n]+\frac{2}{n\pi}sin\frac{n\pi}{2}\ (n=1,2,\cdot\cdot\cdot),\\\\ &\ \ \ \ \ \ \ \ b_n=\int_{-1}^{1}f(x)sin(n\pi x)dx=\int_{-1}^{0}xsin(n\pi x)dx+\int_{0}^{\frac{1}{2}}sin(n\pi x)dx-\int_{\frac{1}{2}}^{1}sin(n\pi x)dx=\\\\ &\ \ \ \ \ \ \ \ -\frac{2}{n\pi}cos\frac{n\pi}{2}+\frac{1}{n\pi}\ (n=1,2,\cdot\cdot\cdot),因为f(x)满足收敛定理的条件,其间断点x=2k,2k+\frac{1}{2},k \in Z,\\\\ &\ \ \ \ \ \ \ \ 所以f(x)=-\frac{1}{4}+\sum_{n=1}^{\infty}\left\{\left[\frac{1-(-1)^n}{n^2\pi^2}+\frac{2}{n\pi}sin\frac{n\pi}{2}\right]cos(n\pi x)+\frac{1}{n\pi}\left(1-2cos\frac{n\pi}{2}\right)sin(n\pi x)\right\},\\\\ &\ \ \ \ \ \ \ \ x \in R,\left\{2k, \ 2k+\frac{1}{2}\ \bigg|\ k \in Z\right\}\\\\ &\ \ (3)\ 函数f(x)的半周期l=3,a_0=\frac{1}{3}\int_{-3}^{3}f(x)dx=\frac{1}{3}\left[\int_{-3}^{0}(2x+1)dx+\int_{0}^{3}dx\right]=-1,\\\\ &\ \ \ \ \ \ \ \ a_n=\frac{1}{3}\int_{-3}^{3}f(x)cos\frac{n\pi x}{3}dx=\frac{1}{3}\left[\int_{-3}^{0}(2x+1)cos\frac{n\pi x}{3}dx+\int_{0}^{3}cos\frac{n\pi x}{3}dx\right]=\frac{6}{n^2\pi^2}[1-(-1)^n]\ (n=1,2,\cdot\cdot\cdot),\\\\ &\ \ \ \ \ \ \ \ b_n=\frac{1}{3}\int_{-3}^{3}f(x)sin\frac{n\pi x}{3}dx=\frac{1}{3}\left[\int_{-3}^{0}(2x+1)sin\frac{n\pi x}{3}dx+\int_{0}^{3}sin\frac{n\pi x}{3}dx\right]=\frac{6}{n\pi}(-1)^{n+1}\ (n=1,2,\cdot\cdot\cdot),\\\\ &\ \ \ \ \ \ \ \ 因为f(x)满足收敛定理的条件,其间断点为x=3(2k+1),k \in Z,\\\\ &\ \ \ \ \ \ \ \ 所以f(x)=-\frac{1}{2}+\sum_{n=1}^{\infty}\left\{\frac{6}{n^2\pi^2}[1-(-1)^n]cos\frac{n\pi x}{3}+(-1)^{n+1}\frac{6}{n\pi}sin\frac{n\pi x}{3}\right\},x \in R,\{3(2k+1)\ |\ k \in Z\}. & \end{aligned} (1) 函数f(x)是半周期l=21的偶函数,所以bn=0 (n=1,2,⋅⋅⋅),a0=4∫021(1−x2)dx=611, an=4∫021(1−x2)cos(2nπx)dx=4[2nπ1−x2sin(2nπx)−4n2π22xcos(2nπx)+8n3π32sin(2nπx)]021= n2π2(−1)n+1 (n=1,2,⋅⋅⋅),因为f(x)满足收敛定理的条件且处处连续, 所以f(x)=1211+π21n=1∑∞n2(−1)n+1cos(2nπx),x∈(−∞, +∞). (2) 函数f(x)的半周期为l=1,a0=∫−11f(x)dx=∫−10xdx+∫021dx+∫211(−1)dx=−21, an=∫−11f(x)cos(nπx)dx=∫−10xcos(nπx)dx+∫021cos(nπx)dx−∫211cos(nπx)dx= [nπxsin(nπx)+n2π21cos(nπx)]−10+[nπ1sin(nπx)]021+[nπ1sin(nπx)]121= n2π21[1−(−1)n]+nπ2sin2nπ (n=1,2,⋅⋅⋅), bn=∫−11f(x)sin(nπx)dx=∫−10xsin(nπx)dx+∫021sin(nπx)dx−∫211sin(nπx)dx= −nπ2cos2nπ+nπ1 (n=1,2,⋅⋅⋅),因为f(x)满足收敛定理的条件,其间断点x=2k,2k+21,k∈Z, 所以f(x)=−41+n=1∑∞{[n2π21−(−1)n+nπ2sin2nπ]cos(nπx)+nπ1(1−2cos2nπ)sin(nπx)}, x∈R,{2k, 2k+21 k∈Z} (3) 函数f(x)的半周期l=3,a0=31∫−33f(x)dx=31[∫−30(2x+1)dx+∫03dx]=−1, an=31∫−33f(x)cos3nπxdx=31[∫−30(2x+1)cos3nπxdx+∫03cos3nπxdx]=n2π26[1−(−1)n] (n=1,2,⋅⋅⋅), bn=31∫−33f(x)sin3nπxdx=31[∫−30(2x+1)sin3nπxdx+∫03sin3nπxdx]=nπ6(−1)n+1 (n=1,2,⋅⋅⋅), 因为f(x)满足收敛定理的条件,其间断点为x=3(2k+1),k∈Z, 所以f(x)=−21+n=1∑∞{n2π26[1−(−1)n]cos3nπx+(−1)n+1nπ6sin3nπx},x∈R,{3(2k+1) ∣ k∈Z}.
2. 将下列函数分别展开成正弦级数和余弦级数: \begin{aligned}&2. \ 将下列函数分别展开成正弦级数和余弦级数:&\end{aligned} 2. 将下列函数分别展开成正弦级数和余弦级数:
( 1 ) f ( x ) = { x , 0 ≤ x < l 2 , l − x , l 2 ≤ x ≤ l . ; ( 2 ) f ( x ) = x 2 ( 0 ≤ x ≤ 2 ) \begin{aligned} &\ \ (1)\ \ f(x)=\begin{cases}x,0 \le x \lt \frac{l}{2},\\\\l-x,\frac{l}{2} \le x \le l.\end{cases};\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ (2)\ \ f(x)=x^2\ (0 \le x \le 2) & \end{aligned} (1) f(x)=⎩ ⎨ ⎧x,0≤x<2l,l−x,2l≤x≤l.; (2) f(x)=x2 (0≤x≤2)
解:
( 1 ) 正弦级数:作 φ ( x ) 为 f ( x ) 的奇延拓,令 Φ ( x ) 是 φ ( x ) 的周期延拓,则 Φ ( x ) 是奇函数,周期为 2 l , Φ ( x ) 满足收敛定理的条件且处处连续,在 [ 0 , l ] 上, Φ ( x ) ≡ f ( x ) , a n = 0 ( n = 0 , 1 , 2 , ⋅ ⋅ ⋅ ) , b n = 2 l [ ∫ 0 l 2 x s i n n π x l d x + ∫ l 2 l ( l − x ) s i n n π x l d x ] ,上式的第二积分中令 l − x = t ,有 ∫ l 2 l ( l − x ) s i n n π x l d x = − ∫ 0 l 2 t c o s n π s i n n π t l d t = ( − 1 ) n − 1 ∫ 0 l 2 t s i n n π t l d t ,则 b n = 2 l [ 1 + ( − 1 ) n − 1 ] ∫ 0 l 2 x s i n n π x l d x ,当 n = 2 k 时, b 2 k = 0 , 当 n = 2 k − 1 时, b 2 k − 1 = 4 l ∫ 0 l 2 x s i n ( 2 k − 1 ) π x l d x = 4 l ( 2 k − 1 ) 2 π 2 ( − 1 ) k − 1 ( k = 1 , 2 , ⋅ ⋅ ⋅ ) , 所以 f ( x ) = 4 l π 2 ∑ n = 1 ∞ ( − 1 ) k − 1 ( 2 k − 1 ) 2 s i n ( 2 k − 1 ) π x l , x ∈ [ 0 , l ] . 余弦级数:作 ψ ( x ) 为 f ( x ) 的偶延拓,令 Ψ ( x ) 是 ψ ( x ) 的周期延拓,则 Ψ ( x ) 是周期为 2 l 的周期函数, Ψ ( x ) 满足收敛定理的条件且处处连续,在 [ 0 , l ] 上, Ψ ( x ) ≡ f ( x ) , a 0 = 2 l [ ∫ 0 l 2 x d x + ∫ l 2 l ( l − x ) d x ] = l 2 , a n = l 2 [ ∫ 0 l 2 x c o s n π x l d x + ∫ l 2 l ( l − x ) c o s n π x l d x ] ,上式第二积分中令 l − x = t ,有 ∫ l 2 l ( l − x ) c o s n π x l d x = ( − 1 ) n ∫ 0 l 2 t c o s n π t l d t ,则 a n = 2 l [ 1 + ( − 1 ) n ] ∫ 0 l 2 x c o s n π x l d x = 2 l [ 1 + ( − 1 ) n ] ( l π ) 2 ( π 2 n s i n n π 2 + 1 n 2 c o s n π 2 − 1 n 2 ) , 当 n = 2 m − 1 时, a 2 m − 1 = 0 , 当 n = 2 m 时, a 2 m = 4 l π 2 ⋅ 1 ( 2 m ) 2 [ ( − 1 ) m − 1 ] = { 0 , m = 2 k , l π 2 ⋅ − 2 ( 2 k − 1 ) 2 , m = 2 k − 1 ( k = 1 , 2 , ⋅ ⋅ ⋅ ) , f ( x ) = l 4 − 2 l π 2 ∑ k = 1 ∞ 1 ( 2 k − 1 ) 2 c o s 2 ( 2 k − 1 ) π x l , x ∈ [ 0 , l ] ( 2 ) 正弦级数:作 φ ( x ) 为 f ( x ) 的奇延拓,令 Φ ( x ) 是 φ ( x ) 的周期延拓,则 Φ ( x ) 是周期为 4 的周期函数, Φ ( x ) 满足收敛定理的条件且除了间断点 x = 2 ( 2 k + 1 ) ( k ∈ Z ) 外处处连续, 在 [ 0 , 2 ) 上, Φ ( x ) ≡ f ( x ) , a n = 0 ( n = 0 , 1 , 2 , ⋅ ⋅ ⋅ ) , b n = 2 2 ∫ 0 2 x 2 s i n n π x 2 d x = [ − 2 n π x 2 c o s n π x 2 ] 0 2 + 4 n π ∫ 0 2 x c o s n π x 2 d x = ( − 1 ) n + 1 8 n π + 8 ( n π ) 2 [ x s i n n π x 2 ] 0 2 + 16 ( n π ) 3 [ c o s n π x 2 ] 0 2 = ( − 1 ) n + 1 8 n π + 16 ( n π ) 3 [ ( − 1 ) n − 1 ] ( n = 1 , 2 , ⋅ ⋅ ⋅ ) , 所以 f ( x ) = 8 π ∑ n = 1 ∞ { ( − 1 ) n + 1 n + 2 n 3 π 2 [ ( − 1 ) n − 1 ] } s i n n π x 2 , x ∈ [ 0 , 2 ) . 余弦级数:作 ψ ( x ) 为 f ( x ) 的偶延拓,令 Ψ ( x ) 是 ψ ( x ) 的周期延拓,则 Ψ ( x ) 是周期为 4 的周期函数, Ψ ( x ) 满足收敛定理的条件且处处连续,在 [ 0 , 2 ] 上, Ψ ( x ) ≡ f ( x ) , a 0 = 2 2 ∫ 0 2 x 2 d x = 8 3 , a n = 2 2 ∫ 0 2 x 2 c o s n π x 2 d x = 2 n π [ x 2 s i n n π x 2 ] 0 2 − 4 n π ∫ 0 2 x s i n n π x 2 d x = 8 ( n π ) 2 [ x c o s n π x 2 − 2 n π s i n n π x 2 ] 0 2 = ( − 1 ) n 16 ( n π ) 2 ( n = 1 , 2 , ⋅ ⋅ ⋅ ) , b n = 0 ( n = 1 , 2 , ⋅ ⋅ ⋅ ) ,所以 f ( x ) = 4 3 + 16 π 2 ∑ n = 1 ∞ ( − 1 ) n n 2 c o s n π x 2 , x ∈ [ 0 , 2 ] . \begin{aligned} &\ \ (1)\ 正弦级数:作\varphi(x)为f(x)的奇延拓,令\Phi(x)是\varphi(x)的周期延拓,则\Phi(x)是奇函数,周期为2l,\\\\ &\ \ \ \ \ \ \ \ \Phi(x)满足收敛定理的条件且处处连续,在[0, \ l]上,\Phi(x) \equiv f(x),a_n=0\ (n=0,1,2,\cdot\cdot\cdot),\\\\ &\ \ \ \ \ \ \ \ b_n=\frac{2}{l}\left[\int_{0}^{\frac{l}{2}}xsin\frac{n\pi x}{l}dx+\int_{\frac{l}{2}}^{l}(l-x)sin\frac{n\pi x}{l}dx\right],上式的第二积分中令l-x=t,有\\\\ &\ \ \ \ \ \ \ \ \int_{\frac{l}{2}}^{l}(l-x)sin\frac{n\pi x}{l}dx=-\int_{0}^{\frac{l}{2}}tcos\ n\pi sin\frac{n\pi t}{l}dt=(-1)^{n-1}\int_{0}^{\frac{l}{2}}tsin\frac{n\pi t}{l}dt,则\\\\ &\ \ \ \ \ \ \ \ b_n=\frac{2}{l}[1+(-1)^{n-1}]\int_{0}^{\frac{l}{2}}xsin\frac{n\pi x}{l}dx,当n=2k时,b_{2k}=0,\\\\ &\ \ \ \ \ \ \ \ 当n=2k-1时,b_{2k-1}=\frac{4}{l}\int_{0}^{\frac{l}{2}}xsin\frac{(2k-1)\pi x}{l}dx=\frac{4l}{(2k-1)^2\pi^2}(-1)^{k-1}\ (k=1,2,\cdot\cdot\cdot),\\\\ &\ \ \ \ \ \ \ \ 所以f(x)=\frac{4l}{\pi^2}\sum_{n=1}^{\infty}\frac{(-1)^{k-1}}{(2k-1)^2}sin\frac{(2k-1)\pi x}{l},x \in [0, \ l].\\\\ &\ \ \ \ \ \ \ \ 余弦级数:作\psi(x)为f(x)的偶延拓,令\Psi(x)是\psi(x)的周期延拓,则\Psi(x)是周期为2l的周期函数,\\\\ &\ \ \ \ \ \ \ \ \Psi(x)满足收敛定理的条件且处处连续,在[0, \ l]上,\Psi(x) \equiv f(x),a_0=\frac{2}{l}\left[\int_{0}^{\frac{l}{2}}xdx+\int_{\frac{l}{2}}^{l}(l-x)dx\right]=\frac{l}{2},\\\\ &\ \ \ \ \ \ \ \ a_n=\frac{l}{2}\left[\int_{0}^{\frac{l}{2}}xcos\frac{n\pi x}{l}dx+\int_{\frac{l}{2}}^{l}(l-x)cos\frac{n\pi x}{l}dx\right],上式第二积分中令l-x=t,有\\\\ &\ \ \ \ \ \ \ \ \int_{\frac{l}{2}}^{l}(l-x)cos\frac{n\pi x}{l}dx=(-1)^n\int_{0}^{\frac{l}{2}}tcos\frac{n\pi t}{l}dt,则\\\\ &\ \ \ \ \ \ \ \ a_n=\frac{2}{l}[1+(-1)^n]\int_{0}^{\frac{l}{2}}xcos\frac{n\pi x}{l}dx=\frac{2}{l}[1+(-1)^n]\left(\frac{l}{\pi}\right)^2\left(\frac{\pi}{2n}sin\frac{n\pi}{2}+\frac{1}{n^2}cos\frac{n\pi}{2}-\frac{1}{n^2}\right),\\\\ &\ \ \ \ \ \ \ \ 当n=2m-1时,a_{2m-1}=0,\\\\ &\ \ \ \ \ \ \ \ 当n=2m时,a_{2m}=\frac{4l}{\pi^2}\cdot\frac{1}{(2m)^2}[(-1)^m-1]=\begin{cases}0,m=2k,\\\\\frac{l}{\pi^2}\cdot\frac{-2}{(2k-1)^2},m=2k-1\end{cases}\ (k=1,2,\cdot\cdot\cdot),\\\\ &\ \ \ \ \ \ \ \ f(x)=\frac{l}{4}-\frac{2l}{\pi^2}\sum_{k=1}^{\infty}\frac{1}{(2k-1)^2}cos\frac{2(2k-1)\pi x}{l},x \in [0, \ l]\\\\ &\ \ (2)\ 正弦级数:作\varphi(x)为f(x)的奇延拓,令\Phi(x)是\varphi(x)的周期延拓,则\Phi(x)是周期为4的周期函数,\\\\ &\ \ \ \ \ \ \ \ \Phi(x)满足收敛定理的条件且除了间断点x=2(2k+1)\ (k \in Z)外处处连续,\\\\ &\ \ \ \ \ \ \ \ 在[0, \ 2)上,\Phi(x) \equiv f(x),a_n=0\ (n=0,1,2,\cdot\cdot\cdot),\\\\ &\ \ \ \ \ \ \ \ b_n=\frac{2}{2}\int_{0}^{2}x^2sin\frac{n\pi x}{2}dx=\left[-\frac{2}{n\pi}x^2cos\frac{n\pi x}{2}\right]_{0}^{2}+\frac{4}{n\pi}\int_{0}^{2}xcos\frac{n\pi x}{2}dx=\\\\ &\ \ \ \ \ \ \ \ (-1)^{n+1}\frac{8}{n\pi}+\frac{8}{(n\pi)^2}\left[xsin\frac{n\pi x}{2}\right]_{0}^{2}+\frac{16}{(n\pi)^3}\left[cos\frac{n\pi x}{2}\right]_{0}^{2}=(-1)^{n+1}\frac{8}{n\pi}+\frac{16}{(n\pi)^3}[(-1)^n-1]\ (n=1,2,\cdot\cdot\cdot),\\\\ &\ \ \ \ \ \ \ \ 所以f(x)=\frac{8}{\pi}\sum_{n=1}^{\infty}\left\{\frac{(-1)^{n+1}}{n}+\frac{2}{n^3\pi^2}[(-1)^n-1]\right\}sin\frac{n\pi x}{2},x \in [0, \ 2).\\\\ &\ \ \ \ \ \ \ \ 余弦级数:作\psi(x)为f(x)的偶延拓,令\Psi(x)是\psi(x)的周期延拓,则\Psi(x)是周期为4的周期函数,\\\\ &\ \ \ \ \ \ \ \ \Psi(x)满足收敛定理的条件且处处连续,在[0, \ 2]上,\Psi(x) \equiv f(x),a_0=\frac{2}{2}\int_{0}^{2}x^2dx=\frac{8}{3},\\\\ &\ \ \ \ \ \ \ \ a_n=\frac{2}{2}\int_{0}^{2}x^2cos\frac{n\pi x}{2}dx=\frac{2}{n\pi}\left[x^2sin\frac{n\pi x}{2}\right]_{0}^{2}-\frac{4}{n\pi}\int_{0}^{2}xsin\frac{n\pi x}{2}dx=\frac{8}{(n\pi)^2}\left[xcos\frac{n\pi x}{2}-\frac{2}{n\pi}sin\frac{n\pi x}{2}\right]_{0}^{2}=\\\\ &\ \ \ \ \ \ \ \ (-1)^n\frac{16}{(n\pi)^2}\ (n=1,2,\cdot\cdot\cdot),b_n=0\ (n=1,2,\cdot\cdot\cdot),所以f(x)=\frac{4}{3}+\frac{16}{\pi^2}\sum_{n=1}^{\infty}\frac{(-1)^n}{n^2}cos\frac{n\pi x}{2},x \in [0, \ 2]. & \end{aligned} (1) 正弦级数:作φ(x)为f(x)的奇延拓,令Φ(x)是φ(x)的周期延拓,则Φ(x)是奇函数,周期为2l, Φ(x)满足收敛定理的条件且处处连续,在[0, l]上,Φ(x)≡f(x),an=0 (n=0,1,2,⋅⋅⋅), bn=l2[∫02lxsinlnπxdx+∫2ll(l−x)sinlnπxdx],上式的第二积分中令l−x=t,有 ∫2ll(l−x)sinlnπxdx=−∫02ltcos nπsinlnπtdt=(−1)n−1∫02ltsinlnπtdt,则 bn=l2[1+(−1)n−1]∫02lxsinlnπxdx,当n=2k时,b2k=0, 当n=2k−1时,b2k−1=l4∫02lxsinl(2k−1)πxdx=(2k−1)2π24l(−1)k−1 (k=1,2,⋅⋅⋅), 所以f(x)=π24ln=1∑∞(2k−1)2(−1)k−1sinl(2k−1)πx,x∈[0, l]. 余弦级数:作ψ(x)为f(x)的偶延拓,令Ψ(x)是ψ(x)的周期延拓,则Ψ(x)是周期为2l的周期函数, Ψ(x)满足收敛定理的条件且处处连续,在[0, l]上,Ψ(x)≡f(x),a0=l2[∫02lxdx+∫2ll(l−x)dx]=2l, an=2l[∫02lxcoslnπxdx+∫2ll(l−x)coslnπxdx],上式第二积分中令l−x=t,有 ∫2ll(l−x)coslnπxdx=(−1)n∫02ltcoslnπtdt,则 an=l2[1+(−1)n]∫02lxcoslnπxdx=l2[1+(−1)n](πl)2(2nπsin2nπ+n21cos2nπ−n21), 当n=2m−1时,a2m−1=0, 当n=2m时,a2m=π24l⋅(2m)21[(−1)m−1]=⎩ ⎨ ⎧0,m=2k,π2l⋅(2k−1)2−2,m=2k−1 (k=1,2,⋅⋅⋅), f(x)=4l−π22lk=1∑∞(2k−1)21cosl2(2k−1)πx,x∈[0, l] (2) 正弦级数:作φ(x)为f(x)的奇延拓,令Φ(x)是φ(x)的周期延拓,则Φ(x)是周期为4的周期函数, Φ(x)满足收敛定理的条件且除了间断点x=2(2k+1) (k∈Z)外处处连续, 在[0, 2)上,Φ(x)≡f(x),an=0 (n=0,1,2,⋅⋅⋅), bn=22∫02x2sin2nπxdx=[−nπ2x2cos2nπx]02+nπ4∫02xcos2nπxdx= (−1)n+1nπ8+(nπ)28[xsin2nπx]02+(nπ)316[cos2nπx]02=(−1)n+1nπ8+(nπ)316[(−1)n−1] (n=1,2,⋅⋅⋅), 所以f(x)=π8n=1∑∞{n(−1)n+1+n3π22[(−1)n−1]}sin2nπx,x∈[0, 2). 余弦级数:作ψ(x)为f(x)的偶延拓,令Ψ(x)是ψ(x)的周期延拓,则Ψ(x)是周期为4的周期函数, Ψ(x)满足收敛定理的条件且处处连续,在[0, 2]上,Ψ(x)≡f(x),a0=22∫02x2dx=38, an=22∫02x2cos2nπxdx=nπ2[x2sin2nπx]02−nπ4∫02xsin2nπxdx=(nπ)28[xcos2nπx−nπ2sin2nπx]02= (−1)n(nπ)216 (n=1,2,⋅⋅⋅),bn=0 (n=1,2,⋅⋅⋅),所以f(x)=34+π216n=1∑∞n2(−1)ncos2nπx,x∈[0, 2].
3. 设 f ( x ) 是周期为 2 的周期函数,它在 [ − 1 , 1 ) 上的表达式为 f ( x ) = e − x ,试将 f ( x ) 展开成 复数形式的傅里叶级数 . \begin{aligned}&3. \ 设f(x)是周期为2的周期函数,它在[-1, \ 1)上的表达式为f(x)=e^{-x},试将f(x)展开成\\\\&\ \ \ \ 复数形式的傅里叶级数.&\end{aligned} 3. 设f(x)是周期为2的周期函数,它在[−1, 1)上的表达式为f(x)=e−x,试将f(x)展开成 复数形式的傅里叶级数.
解:
f ( x ) 满足收敛定理的条件,且除了点 x = 2 k + 1 ( k ∈ Z ) 外处处连续, c n = 1 2 ∫ − 1 1 e − x e − i n π x d x = 1 2 ∫ − 1 1 e − ( 1 + n π i ) x d x = − 1 2 ⋅ 1 1 + n π i [ e − ( 1 + n π i ) x ] − 1 1 = − 1 2 ⋅ 1 − n π i 1 + ( n π ) 2 ( e − 1 ⋅ e − n π i − e ⋅ e n π i ) = 1 − n π i 1 + ( n π ) 2 ⋅ e c o s n π − e − 1 c o s n π 2 = ( − 1 ) n e − e − 1 2 1 − n π i 1 + n 2 π 2 ( n = 0 , ± 1 , ± 2 , ⋅ ⋅ ⋅ ) , 所以 f ( x ) = ∑ n = − ∞ ∞ ( − 1 ) n e − e − 1 2 1 − n π i 1 + n 2 π 2 ⋅ e i n π x , x ∈ R , { 2 k + 1 ∣ k ∈ Z } . \begin{aligned} &\ \ f(x)满足收敛定理的条件,且除了点x=2k+1\ (k \in Z)外处处连续,\\\\ &\ \ c_n=\frac{1}{2}\int_{-1}^{1}e^{-x}e^{-in\pi x}dx=\frac{1}{2}\int_{-1}^{1}e^{-(1+n\pi i)x}dx=-\frac{1}{2}\cdot\frac{1}{1+n\pi i}[e^{-(1+n\pi i)x}]_{-1}^{1}=-\frac{1}{2}\cdot\frac{1-n\pi i}{1+(n\pi)^2}(e^{-1}\cdot e^{-n\pi i}-e\cdot e^{n\pi i})=\\\\ &\ \ \frac{1-n\pi i}{1+(n\pi)^2}\cdot \frac{ecos\ n\pi-e^{-1}cos\ n\pi}{2}=(-1)^n\frac{e-e^{-1}}{2}\frac{1-n\pi i}{1+n^2\pi^2}\ (n=0,\pm1,\pm2,\cdot\cdot\cdot),\\\\ &\ \ 所以f(x)=\sum_{n=-\infty}^{\infty}(-1)^n\frac{e-e^{-1}}{2}\frac{1-n\pi i}{1+n^2\pi^2}\cdot e^{in\pi x},x \in R,\{2k+1\ |\ k \in Z\}. & \end{aligned} f(x)满足收敛定理的条件,且除了点x=2k+1 (k∈Z)外处处连续, cn=21∫−11e−xe−inπxdx=21∫−11e−(1+nπi)xdx=−21⋅1+nπi1[e−(1+nπi)x]−11=−21⋅1+(nπ)21−nπi(e−1⋅e−nπi−e⋅enπi)= 1+(nπ)21−nπi⋅2ecos nπ−e−1cos nπ=(−1)n2e−e−11+n2π21−nπi (n=0,±1,±2,⋅⋅⋅), 所以f(x)=n=−∞∑∞(−1)n2e−e−11+n2π21−nπi⋅einπx,x∈R,{2k+1 ∣ k∈Z}.
4. 设 u ( t ) 是周期为 T 的周期函数,已知它的傅里叶级数的复数形式为(参阅本节例题) u ( t ) = h τ T + h π ∑ n = − ∞ n ≠ 0 ∞ 1 n s i n n π τ T e 2 n π t T i ( − ∞ < t < + ∞ ) , 试写出 u ( t ) 的傅里叶级数的实数形式(即三角形式) . \begin{aligned}&4. \ 设u(t)是周期为T的周期函数,已知它的傅里叶级数的复数形式为(参阅本节例题)\\\\&\ \ \ \ u(t)=\frac{h\tau}{T}+\frac{h}{\pi}\sum_{n=-\infty\ \ n \neq 0}^{\infty}\frac{1}{n}sin\frac{n\pi \tau}{T}e^{\frac{2n\pi t}{T}i}\ (-\infty \lt t \lt +\infty),\\\\&\ \ \ \ 试写出u(t)的傅里叶级数的实数形式(即三角形式).&\end{aligned} 4. 设u(t)是周期为T的周期函数,已知它的傅里叶级数的复数形式为(参阅本节例题) u(t)=Thτ+πhn=−∞ n=0∑∞n1sinTnπτeT2nπti (−∞<t<+∞), 试写出u(t)的傅里叶级数的实数形式(即三角形式).
解:
已知 c n = h n π s i n n π τ T ( n = ± 1 , ± 2 , ⋅ ⋅ ⋅ ) ,因为 c n = a n − i b n 2 , c − n = a n + i b n 2 = c n ‾ ( n = 1 , 2 , ⋅ ⋅ ⋅ ) , 则 a n = R e ( 2 c n ) , b n = I m ( 2 c n ‾ ) ,因 c n 为实数,所以 a n = 2 h n π s i n n π τ T , b n = 0 ( n = 1 , 2 , ⋅ ⋅ ⋅ ) , 所以 u ( t ) = h τ T + 2 h π ∑ n = 1 ∞ 1 n s i n n π τ T ⋅ c o s 2 n π t T ( − ∞ < t + ∞ ) . \begin{aligned} &\ \ 已知c_n=\frac{h}{n\pi}sin\frac{n\pi \tau}{T}\ (n=\pm1,\pm2,\cdot\cdot\cdot),因为c_n=\frac{a_n-ib_n}{2},c_{-n}=\frac{a_n+ib_n}{2}=\overline{c_n}\ (n=1,2,\cdot\cdot\cdot),\\\\ &\ \ 则a_n=Re(2c_n),b_n=Im(2\overline{c_n}),因c_n为实数,所以a_n=\frac{2h}{n\pi}sin\frac{n\pi \tau}{T},b_n=0\ (n=1,2,\cdot\cdot\cdot),\\\\ &\ \ 所以u(t)=\frac{h\tau}{T}+\frac{2h}{\pi}\sum_{n=1}^{\infty}\frac{1}{n}sin\frac{n\pi \tau}{T}\cdot cos\frac{2n\pi t}{T}\ (-\infty \lt t +\infty). & \end{aligned} 已知cn=nπhsinTnπτ (n=±1,±2,⋅⋅⋅),因为cn=2an−ibn,c−n=2an+ibn=cn (n=1,2,⋅⋅⋅), 则an=Re(2cn),bn=Im(2cn),因cn为实数,所以an=nπ2hsinTnπτ,bn=0 (n=1,2,⋅⋅⋅), 所以u(t)=Thτ+π2hn=1∑∞n1sinTnπτ⋅cosT2nπt (−∞<t+∞).
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