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1．推导直线的夹角公式设直线l1:A1x+B1y+C1=0与l2:A2x+ B2y+C2=0,两直线的夹角为α,两直线方向向量的夹角为θ,则α=θ或α=π-θ．因为两直线的方向向量分别为
1. Derivation of the straight line between the angle formula to set the line l1: A1x + B1y + C1 = 0 and l2: A2x + B2y + C2 = 0, the angle between the two lines is α, the angle between the two linear direction of the vector is θ, then α = θ or α=π-θ. Because the directions of the two straight lines are