The expression xchk == X3 is indeed false, however , if you check the value of abs(xchk-X3) you will find it to be less than machine precision. Since you are dealing with floating point representations, a more robust way of checking the equality of two scalar quantities, A and B, would be
abs(A-B)<tol
where 'tol' is some tolerance parameter (e.g. tol=eps)
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