Cho hai mặt phẳng (alpha), (beta). Phát biểu nào sau đây

Nếu $\left( \alpha \right)$ cắt $\left( \beta \right)$ thì $\left( \alpha \right) \bot \left( \beta \right)$.

Nếu $\left( {\left( \alpha \right),\left( \beta \right)} \right) = 0^\circ $ thì $\left( \alpha \right) \bot \left( \beta \right)$.

Nếu $\left( {\left( \alpha \right),\left( \beta \right)} \right) = 45^\circ $ thì $\left( \alpha \right) \bot \left( \beta \right)$.

Nếu $\left( {\left( \alpha \right),\left( \beta \right)} \right) = 90^\circ $thì $\left( \alpha \right) \bot \left( \beta \right)$.