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例1 如图1.在Rt△ABC中,∠A-90°,⊙O 分别与AB、AC相切于点E、F,圆心O在BC上.若 AB=a,AC=b,则⊙O半径等于( ).
Example 1 As shown in Figure 1. In Rt △ ABC, ∠A-90°, ⊙O are tangent to AB and AC at points E and F respectively, and the center of circle O is on BC. If AB=a and AC=b, then O radius is equal to ().