C为(2t,t),CA = (1-2t, 7-t),CB = (5-2t, 1-t).CA?CB = (1-2t)(5-2t) + (7-t)(1-t) = 4t^2 -12t + 5 + t^2 -8t +7 = 5t^2 - 20t + 12 = 5(t^2 - 4t + 4) - 8=5(t-2)^2 - 8>=-8.t=2时，CA?CB取得最小值-8.此时，OC = (2t, t) = (4,2). CA = (1-2t, 7-t)=(-3,6), |CA| = [3^2 + 6^2]^(1/2) = 3[ 1 + 4]^(1/2) = 3*5^(1/2)CB = (5-2t, 1...
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