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$  (x-x_{a})\begin{vmatrix} y_{p} & z_{p}\\ y_{q} & z_{q} \end{vmatrix}+ (y-y_{a})\begin{vmatrix} z_{p} & x_{p}\\ z_{q} & x_{q} \end{vmatrix}+ (z-z_{a})\begin{vmatrix} x_{p} & y_{p}\\ x_{q} & y_{q} \end{vmatrix}=0  $
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