B X(x)=C1cos√Ax+C2sin√x X(0)=0和X(=0→G=0和c2 cos√Al=0 (k+)2丌 (2k+1)2x k=0,1,2,3 4 X(x)=C2sn C.r+(2k+1)nar=0 2k +D)rat T(=Acos Bsin (2k+1)rat 27 u,(x, t)=(A, cos (2k+lrat B (2k+1)mt1(2k+1) k=0,1,2,3 2l 27 K=0:基频 K>0:谐频X (x) C cos  x C sin  x = 1 + 2 B. X (0) = 0 和 X '(l) = 0.  C1 = 0 cos 0 和 C2 l = 2 2 2 2 2 2 4 (2 1) ) 2 1 ( l k l k    + = + = k = 0,1,2,3 l k x X x C 2 (2 1) ( ) sin 2 +  = C. 0; 4 (2 1) '' 2 2 2 2 = + + T l k a T  , 2 (2 1) sin 2 (2 1) ( ) cos l k at B l k at T t A  +  + + = . 2 (2 1) )sin 2 (2 1) sin 2 (2 1) ( , ) ( cos l k x l k at B l k at uk x t Ak k  +  +  + + = k = 0,1,2,3 K=0：基频。 K>0：谐频