[线性代数] 怎么就是得不出正确结果呢？

program S_equation
implicit none
integer::i,j,k,imax,t
real::max,n

!用矩阵（实型动态数组）将线性方程组表示出来!

real,dimension(:,:),allocatable::a,m
real,dimension(:),allocatable::x
write(*,*)'请输入线性方程组的元数n:'
read(*,*)t

!给动态数组分配内存!

allocate(a(t,t+1),m(t,t+1),x(t))
write(*,*)'请依次按行输入线性方程组的增广矩阵a：'
read(*,*)((a(i,j),j=1,t+1),i=1,t)

    do k=1,t-1

    !选取每列的最大列元素!

    max=abs(a(k,k))
    imax=k
    do i=k+1,t
        if(abs(a(i,k)).gt.max)then
        max=abs(a(i,k))
        imax=i
        end if
    end do

    !将最大列元素所在的行与第K行进行交换!

    do j=k,t+1
        m(k,j)=a(k,j)
        a(k,j)=a(imax,j)
        a(imax,j)=m(k,i)
    end do

    !对方程组按X1,X2，...，Xt的顺序进行依次消元，将增广矩阵化为上三角矩阵!

    do i=k+1,t
        m(i,k)=a(i,k)/a(k,k)
        do j=k+1,t+1
            a(i,j)=a(i,j)-m(i,k)*a(k,j)
        end do
    end do
    end do

      !先算出Xt!

    x(t)=a(t,t+1)/a(t,t)

    !回带!

    do k=t-1,1,-1
        n=0
        do j=t,k+1,-1
            n=n+a(k,j)*x(j)
        end do
        x(k)=(a(k,t+1)-n)/a(k,k)
    end do

    write(*,*)'线性方程组的解为：'
    do i=1,t
        write(*,*)x(i)
    end do
    read(*,*)
    end program S_equation

program S_equation  implicit none
  integer::i,k,imax,t
  real::max,n
  !用矩阵（实型动态数组）将线性方程组表示出来!
  real,dimension(:,:),allocatable::a,b
  real,dimension(:),allocatable::x,m
  write(*,*)'请输入线性方程组的元数n:'
  read(*,*)t
  !给动态数组分配内存!
  allocate(a(t,t+1),m(t+1),x(t),b(t,t+1))
  write(*,*)'请依次按行输入线性方程组的增广矩阵a：'
  read(*,*)( b(i,:),i=1,t)
  a=b
  do k=1,t-1
    !选取每列的最大列元素!
    max=maxval(a(k:,k))!abs(a(k,k))
    imax=maxloc(a(k:,k),1) + k - 1    
    !将最大列元素所在的行与第K行进行交换!
    m(k:)    = a(k,k:)
    a(k,k:)    = a(imax,k:)
    a(imax,k:) = m(k:)
    !对方程组按X1,X2，...，Xt的顺序进行依次消元，将增广矩阵化为上三角矩阵!
    do i=k+1,t
      a(i,k+1:)=a(i,k+1:) - a(i,k)/a(k,k)*a(k,k+1:)
    end do
  end do
  !先算出Xt!
  x(t)=a(t,t+1)/a(t,t)
  !回带!
  do k=t-1,1,-1
    n= sum( a(k,k+1:t)*x(k+1:) )
    x(k)=(a(k,t+1)-n)/a(k,k)
  end do
  write(*,*)'线性方程组的解为：'
  write(*,*) x
  write(*,*) matmul( b(1:t,1:t) , reshape(x,[t,1]) )!//验证
  read(*,*)
end program S_equation