SUBROUTINE TRISYMM ( MXN, NNODE, A , B )
      IMPLICIT REAL*8 ( A-H , O-Z )
      DIMENSION A(MXN,2) , B(MXN)
      B(1) = B(1) / A(1,1)
      A(1,1) = A(1,2) / A(1,1)
      DO I = 2 , NNODE
      P = A(I,1) - A(I-1,2) * A(I-1,1)
      A(I,1) = A(I,2) / P
      B(I) = ( B(I) - A(I-1,2)*B(I-1) ) / P
      END DO
C------ BACK SUBSTITUTION ----
      DO I = NNODE-1, 1,-1
      B(I) = B(I) - A(I,1) * B(I+1)
      END DO
      RETURN
      END