A particle is in equilibrium under the action of three forces F1, F2 and F3 (unit: Newton) on the plane. Given that F1 and F2 form an angle of 60 ° and the sizes of F1 and F2 are 2 and 4 respectively, the size of F3 is 0______ ．

As shown in the figure, according to the meaning of the question, ab = 2, BC = 4, ∠ ABC = 120 ° according to the cosine theorem, ac2 = 22 + 42-2 × 2 × 4 × cos120 ° = 28 ∧ AC = 27, so the answer is 27

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