Có: `\hat{xOy}=\hat{mOn}=90^@` (`2` góc đối đỉnh)

Lại có: `\hat{xOy}+\hat{mOn}+\hat{xOm}+\hat{yOn}=360^@`

           `90^@+90^@+\hat{xOm}+\hat{yOn}=360^@`

           `180^@+\hat{xOm}+\hat{yOn}=360^@`

           `\hat{xOm}+\hat{yOn}=360^@-180^@`

           `\hat{xOm}+\hat{yOn}=180^@`

mà `\hat{xOm}=\hat{yOn}` (`2` góc đối đỉnh)

`=> \hat{xOm}=\hat{yOn}=180^@ :2=90^@`

Vậy `\hat{mOn}=\hat{xOm}=\hat{yOn}=90^@`