Góc \(\phi (0^o \le \phi \le 90^o)\) giữa 2 đường thẳng:
\(Ax+By+Cy=0 \) và \(Ax' + By' + C' = 0\)
\(cos\phi = \dfrac{\vert{\vec{n}.\vec{n'}}\vert}{\vert{\vec{n}}\vert.\vert{\vec{n'}\vert}} = \dfrac{\vert{AA' + BB'}\vert}{\sqrt{A^2 + B^2}. \sqrt{A'^2 + B'^2}}\)
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