Let $P$ be the plane passing through the origin with normal vector $\begin{pmatrix} 1 \\ -2 \\ 1 \end{pmatrix}.$  Find the matrix $\mathbf{P}$ such that for any vector $\mathbf{v},$ $\mathbf{P} \mathbf{v}$ is the projection of $\mathbf{v}$ onto plane $P.$
Question:
Let $P$ be the plane passing through the origin with normal vector $\begin{pmatrix} 1 \\ -2 \\ 1 \end{pmatrix}.$  Find the matrix $\mathbf{P}$ such that for any vector $\mathbf{v},$ $\mathbf{P} \mathbf{v}$ is the projection of $\mathbf{v}$ onto plane $P.$
Answer:
Let's think step by step.

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\begin{pmatrix} \frac{5}{6} & \frac{1}{3} & -\frac{1}{6} \\ \frac{1}{3} & \frac{1}{3} & \frac{1}{3} \\ -\frac{1}{6} & \frac{1}{3} & \frac{5}{6} \end{pmatrix}
Let $P$ be the plane passing through the origin with normal vector $\begin{pmatrix} 1 \\ -2 \\ 1 \end{pmatrix}.$  Find the matrix $\mathbf{P}$ such that for any vector $\mathbf{v},$ $\mathbf{P} \mathbf{v}$ is the projection of $\mathbf{v}$ onto plane $P.$
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