Either Xb < tYa (Case 1), or else Xb tYa (Case 2).

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Case 1 (Xb < tYa):  Let Xa be any positive quantity less than Xb.

Let Yb = Xa/t. (Thus t = Xa/Yb.)

Therefore 0 < Yb = Xa/t < Xb/t < (tYa)/t = Ya.

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Case 2 (Xb tYa):  Let Yb be any positive quantity less than Ya.

Let Xa = tYb. (Thus t = Xa/Yb.)

Therefore 0 < Xa = tYb < tYa Xb.