Cho hình lập phương \(A B C D \cdot A^{\prime} B^{...

A

- Hướng dẫn giải

\(\begin{aligned} &\left\{\begin{array}{cc} A^{\prime} D \perp A D^{\prime} & (t / c H V) \\ A^{\prime} D \perp C^{\prime} D^{\prime} & \left(C^{\prime} D^{\prime} \perp\left(A^{\prime} D^{\prime} D A\right)\right) \end{array}\right.\\ &\Rightarrow A^{\prime} D \perp\left(A C^{\prime} D^{\prime}\right) \Rightarrow A^{\prime} D \perp A C^{\prime}\\ &\left\{\begin{array}{lr} A^{\prime} B \perp A B^{\prime} & (t / c H V) \\ A^{\prime} B \perp B^{\prime} C^{\prime} & \left(B^{\prime} C^{\prime} \perp\left(A^{\prime} D^{\prime} D A\right)\right) \end{array}\right.\\ &\Rightarrow A^{\prime} B \perp\left(A B^{\prime} C^{\prime}\right) \Rightarrow A^{\prime} B \perp A C^{\prime}\\ &\text { Từ }(1),(2) \Rightarrow A C^{\prime} \perp\left(A^{\prime} B D\right) \end{aligned}\)