I want to know about properties of non-integer combination, i.e. given a,b,...∈Z; but the multipliers x,y,.. not all ∈ Z. I hope that they must be enjoying similar properties, as they are comprised of rationals, and the rationals are closed under addition too.

If so, then how the property (of closure of rationals under addition) can be used where the linear combinations do not hold.

A familiar case is in g.c.d. computation, where the invariant property (g.c.d. is same at each step) is a product of two linear equations being followed at each step that lead to common divisors of two pairs: (i) remainder (r), divisor(a), & (ii) dividend (d), and divisor(a). The Euclid algorithm reduces the quantities of dividend(d), divisor(a) at each step, while keeping the invariant property being followed. In (i) & (ii), d & r are the linear combinations respectively. Quotient (q) and divisor (a) are not linear combinations as when taken on l.h.s. lead to r.h.s. side expressions of d−r/a & d−r/q respectively.

Definitely the expression given by d-r/q or d-r/a is a rational expression, and rationals are closed w.r.t. to the addition operation. I want to know, as curiosity, what properties are enjoyed by the two quantities that are not linear combination of integers.