y+2Y=0Y(0)=Y()=0 n丌 Y, ()=A,sin X"+(x2-1)X=0 (0)=X()=0 (m2+n2) ma X,(x)=A,sin v (x,y)=X(xr(y nzx v=E Sinl n  = l n y Yn y An  ( ) = sin 2 2 2 2 2 l m   −  = 2 2 2 2 2 ( ) l m n   + = l m x Xn x An  ( ) = sin l n y l m x vmn Emn   = sin sin n =1,2,  m =1,2,  '' 0 2 Y + Y = Y(0) = Y(l) = 0 '' ( ) 0 2 2 X +  −  X = X (0) = X (l) = 0 v(x, y) = X (x)Y( y)