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用余弦定理证明 \vec u * \vec v = u_1 v_1 + u_2 v_2 = |\vec u||\vec v|costheta 。
用余弦定理证明 \vec u * \vec v = u_1 v_1 + u_2 v_2 = |\vec u||\vec v|costheta 。
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