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ㄧ、 求 下 列 無 窮 級 數 之 斂 散 性<br />
1.<br />
∞<br />
∑<br />
n=0<br />
(-1) n<br />
n+1<br />
2.<br />
∞<br />
∑<br />
n=0<br />
( ) n+1 2<br />
-1 n<br />
(n+2)!<br />
3.<br />
∞<br />
∑<br />
n=0<br />
( ) n<br />
-1 n!<br />
(2n+1)!<br />
4.<br />
∞<br />
∑<br />
n=0<br />
(-1) n<br />
1+ n<br />
5.<br />
6.<br />
7.<br />
8.<br />
9.<br />
10.<br />
∞<br />
∑<br />
n = 1<br />
∞<br />
∑<br />
n = 0<br />
∞<br />
∑<br />
n = 1<br />
∞<br />
∑<br />
n=<br />
1<br />
∞<br />
∑<br />
n=<br />
1<br />
∞<br />
∑<br />
sin ( π n/2)<br />
n<br />
cosπ<br />
n<br />
n+2<br />
n<br />
(-1) ln<br />
n<br />
cosπ<br />
n<br />
n<br />
n<br />
( −1)<br />
n<br />
n!<br />
n!<br />
( 2) n<br />
n= 1 −<br />
n<br />
n<br />
=<br />
<strong>二</strong> 、 求 下 列 函 數 之 Maclaurin series? 並 求 其 收 斂 半 徑<br />
1<br />
1.<br />
1+<br />
x<br />
x<br />
2. 1 + x<br />
2<br />
3.<br />
2<br />
1−<br />
x<br />
4. ln(1 + x)<br />
−1<br />
5. tan x<br />
6. x + 1<br />
7. xsin 2x<br />
2<br />
8. sin x
Sol:<br />
1.<br />
2.<br />
n+<br />
1<br />
( −1)<br />
n<br />
∞<br />
(-1)<br />
2<br />
1 1<br />
lim n +<br />
n+ n+<br />
∑ = = lim ( − 1) Q < 1故 收 斂<br />
n<br />
n=0 n+1 n→∞<br />
( − 1) n→∞<br />
n+ 2 n+<br />
2<br />
n + 1<br />
n+<br />
2 2<br />
( − 1) ( n + 1)<br />
n+1<br />
∞<br />
2 2 2<br />
(-1)<br />
n ( n + 3)! ( − 1)( n + 2n+ 1) n + 2n+<br />
1<br />
∑ =lim = lim Q < 1故 收 斂<br />
n 1 2 2 2<br />
n=0 (n+2)! n→∞<br />
+<br />
( − 1) n n→∞<br />
( n+ 3) n ( n+<br />
3) n<br />
( n + 2)!<br />
n+<br />
1<br />
( − 1) ( n + 1)!<br />
n<br />
∞<br />
(-1)<br />
n! (2n<br />
+ 3)! ( − 1)( n+ 1) n+<br />
1<br />
3. ∑ =lim = lim Q < 1故 收 斂<br />
n<br />
n=0 (2n+1)! n→∞<br />
( − 1) n!<br />
n→∞<br />
(2n+ 2)(2n+ 3) (2n+ 2)(2n+<br />
3)<br />
(2n<br />
+ 1)!<br />
4.<br />
5<br />
n+<br />
1<br />
( −1)<br />
n<br />
∞<br />
(-1)<br />
1 1 ( 1)(1 ) 1<br />
=lim<br />
+ n + − + n + n<br />
∑ = lim Q < 1故 收 斂<br />
n<br />
n=0 1+ n<br />
n→∞<br />
( − 1) n→∞<br />
1+ n+ 1 1+ n+<br />
1<br />
1+<br />
n<br />
∞<br />
∑<br />
n+<br />
1<br />
( −1)<br />
∞ n<br />
sin ( π n/2) ( − 1) 2 3<br />
2 1 2 1<br />
lim n<br />
n+ n+<br />
= +<br />
∑ = = lim ( − 1) Q < 1故 收 斂<br />
n<br />
n 2n+ 1 n→∞<br />
( −1)<br />
n→∞<br />
2n+ 3 2n+<br />
3<br />
2n<br />
+ 1<br />
n = 1 n=<br />
0<br />
.<br />
∞<br />
sin ((n+1) π /2)<br />
ps. ∑<br />
n=1 n=2 n=3 n=4 n=5 n=6<br />
n = 0 n+1<br />
1 1 1<br />
0 - 0 0 -<br />
3 5 7<br />
∞ n<br />
(-1)<br />
∑ 零 項 次 不 加<br />
2n+1<br />
n = 0<br />
6.<br />
∞<br />
n+<br />
1<br />
( −1)<br />
∞ n<br />
cosπ<br />
n ( −1) 3 ( 1)( 2) 2<br />
= lim n<br />
− n+ n+<br />
= + = lim < 1<br />
n<br />
n+2 n+ 2 n→∞<br />
( −1)<br />
n→∞<br />
( n+ 3) n+<br />
3<br />
n + 2<br />
∑ ∑ Q 故 收 斂<br />
n = 0 n=0<br />
n+<br />
1<br />
( − 1) ln( n + 1)<br />
∞ n<br />
(-1) ln n ln( 1) ln( 1)<br />
7.<br />
=lim n + 1<br />
n n+ n n+<br />
∑ = lim ( − 1) Q < 1且 趨 近 於 0, 故 收 斂<br />
n<br />
n = 1 n n→∞<br />
( − 1) lnn n→∞<br />
( n+ 1)lnn n+<br />
1 lnn<br />
n<br />
8.<br />
∞<br />
n+<br />
1<br />
( −1)<br />
∞ n<br />
cosπ<br />
n ( − 1)<br />
= lim<br />
1 n<br />
=<br />
n +<br />
< 1<br />
n<br />
n<br />
n n<br />
→∞ ( − 1) n+<br />
1<br />
n<br />
∑ ∑ Q 故 收 斂<br />
n=<br />
1 n=1
n+ 1 n+<br />
1<br />
( − 1) ( n + 1)<br />
∞ n n n n<br />
( − 1) n ( n + 1)!<br />
( n+ 1) ( n+<br />
1)<br />
9. ∑ = lim = lim ( − 1) Q = e > 1( 發 散 )<br />
n n n n<br />
1 ! n ( 1)<br />
n<br />
n n →∞ n<br />
→∞<br />
=<br />
−<br />
n n<br />
n!<br />
10.<br />
∞<br />
∑<br />
n=<br />
1<br />
( n + 1)!<br />
n+<br />
1<br />
n! ( −2)<br />
n+<br />
1<br />
= lim = lim > 1故 發 散<br />
n<br />
( −2) n→∞<br />
n!<br />
n→∞<br />
−2<br />
n<br />
( −2)<br />
1.<br />
∞<br />
1 1<br />
n<br />
= = ∑( −1)<br />
x<br />
+ x<br />
2<br />
1 1-(-x )<br />
2<br />
=> <<br />
n=<br />
0<br />
n+ 1 2n+<br />
2<br />
( −1)<br />
x<br />
lim < 1<br />
n→∞<br />
n 2n<br />
( −1)<br />
x<br />
x<br />
1<br />
2n<br />
2.<br />
∞<br />
x 1<br />
= x* = x∑( − 1) x = ∑( −1)<br />
x<br />
1+ x 1+<br />
x<br />
x<br />
x<br />
∞<br />
n n n n+<br />
1<br />
n= 0 n=<br />
0<br />
2n+<br />
2<br />
2<br />
lim < 1 => x < 1, 即 x < 1<br />
n→∞<br />
2n<br />
即<br />
x<br />
< 1<br />
3.<br />
2<br />
2 1<br />
= 2( ) = 2<br />
2 2<br />
1−x<br />
1−x<br />
x<br />
x<br />
n=<br />
1<br />
2n<br />
2n+<br />
2<br />
2<br />
lim < 1 => x < 1即<br />
x < 1<br />
n→∞<br />
2n<br />
∞<br />
∑<br />
x<br />
2 3<br />
∞ n<br />
x x ( −1)<br />
ln(1 + x) = x− + + .... = ∑ x<br />
2 3 n + 1<br />
n+<br />
1<br />
4. ( −1)<br />
n+<br />
2<br />
x<br />
lim n + 2 < 1, => x < 1<br />
n→∞<br />
n<br />
( −1)<br />
n+<br />
1<br />
x<br />
n + 1<br />
5. 6.<br />
3 5<br />
∞ n<br />
− 1 x x ( −1)<br />
2n+<br />
1<br />
tan x = x− + + ..... = ∑ x<br />
1<br />
3 5 n=<br />
0 2n<br />
+ 1<br />
1 1 1 1<br />
2<br />
2<br />
x+ 1 = (1 + x) = 1 + x+ ( )( − 1) x + ..<br />
2n+<br />
3<br />
x<br />
2 2! 2 2<br />
2<br />
2 3 (2n<br />
1) x<br />
lim n + +<br />
1 1 2 1 3 5 4<br />
= lim = 1 + x− x + x − x + ...<br />
n→∞<br />
2n+<br />
1<br />
x<br />
n→∞<br />
(2 n + 3)<br />
2 8 16 128<br />
2n<br />
+ 1<br />
x < 1<br />
x<br />
7.<br />
< 1, 即 x < 1<br />
z<br />
xsin 2 x令 Z = x,<br />
x=<br />
2<br />
∞ n<br />
z z ( −1) 1 ( −1)<br />
sin Z = ∑ Z = ∑ Z<br />
2 2 (2n+ 1)! 2 (2n+<br />
1)!<br />
n= 0 n=<br />
0<br />
∞ n<br />
2n+ 1 2n+<br />
2<br />
1 2n+<br />
4<br />
Z<br />
(2n<br />
+ 5)!<br />
1<br />
1<br />
Z<br />
(2n+ 3)(2n+<br />
2)<br />
(2n<br />
+ 1)!<br />
2<br />
lim = lim * < 1<br />
n→∞<br />
2n+<br />
2 n→∞<br />
收 斂 半 徑 −∞< Z
1<br />
x= − x<br />
2<br />
2 4 6<br />
x x x<br />
而 cos x = 1 − + − + ...<br />
2! 4! 6!<br />
以 2 x代 替 x<br />
2<br />
sin (1 cos 2 )<br />
2 4 6<br />
(2 x) (2 x) (2 x)<br />
cos 2x<br />
= 1 − + − + ...<br />
2! 4! 6<br />
2 4 6<br />
2 2 2 4 2 6<br />
= 1 − x + x − x + ...<br />
2! 4! 6!<br />
1<br />
x= − x<br />
2<br />
2 4 6<br />
1 2 2 2 4 2<br />
= (1 − 1 + x − x + −...)<br />
2 2! 4! 6!<br />
2 4 6<br />
1 2 2 2 4 2 6<br />
= ( x − x + x −...)<br />
2 2! 4! 6!<br />
3 5<br />
2 2 2 4 2 6<br />
= x − x + x −...<br />
2! 4! 6!<br />
收 斂 半 徑 為 −∞< z