[Toán 10] Bài tập về tích vô hướng

L

linh123658

Ta có:
$BB_1$vuông góc với $CC_1$nên:
$\vec{BB_1}.\vec{CC_1}=0$
\Leftrightarrow$(\vec{BA}+\vec{AB_1})(\vec{CA}+\vec{AC_1})=0$
\Leftrightarrow$\vec{BA}.\vec{CA}+\vec{BA}.\vec{AC_1}+\vec{AB_1}.\vec{CA}+\vec{AB_1}.\vec{AC_1}=0$
\Leftrightarrow$AB.AC.$cos$A$$-\dfrac{1}{2}AC^2-\dfrac{1}{2}AB^2+\dfrac{1}{4}AB.AC.$cos$A$
\Leftrightarrowcos$A=\dfrac{4}{5}$
$.......................................$
 
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